102

Measuring motion4.1

Motion is movement. Many objects around ushave motion. We can determine the speed ofmoving objects. Motion can be measured.

Before measuring motion we need to updateour knowledge of some words. Distance is howfar it is between two places. Distance is measuredin metres, or its multiples. Metre is abbreviated tom. Small distances are measured in millimetres,abbreviated to mm. Large distances are measuredin kilometres, km. There are many other multiples,such as M, μ and n. Their meanings are shown inthe table.

Multiple Index Number Name Symbolnotation

1 000 000 000 109 billion giga G1 000 000 106 million mega M1000 103 thousand kilo k100 102 hundred hecto h10 101 ten deca D1 100

0.1 10–1 tenth deci d0.01 10–2 hundredth centi c0.001 10–3 thousandth milli m0.000 001 10–6 millionth micro μ0.000 000 001 10–9 billionth nano n

The distances used in this activity are measuredwith a tape measure or a trundle wheel. In atrundle wheel, the circumference (= distancearound) the wheel is one metre. A clicker makescounting easy as you walk along. Each clickmeans you have gone one metre. Some trundlewheels have a counter on them.

Your measurements of distance need to be accu-rate to achieve accurate measurements of speed.

Time is a measure of how long it takes betweentwo events. Time is measured in seconds, minutes, hours, days and weeks. In our experiments time is best measured in seconds. Astopwatch is best for this.

Speed is how fast an object is moving, or thedistance travelled in the time taken. In a car,speed is measured in kilometres per hour, abbreviated to km/h, where the / sign meansdivided by. Most scientists measure speed inmetres per second. This is abbreviated to m/s. Theunit of speed can also be written as m s–1, wherethe index –1 means divided by.

Velocity is speed in a given direction. Directionis important for ships at sea, for aircraft, and forinter-planetary space probes. Velocity tells thespeed and the direction.

The speed of a car changes during its journey.Sometimes the car will stop, or slow to go arounda corner, or travel fast on a highway. Thespeedometer of the car shows the speed of the carat the instant that you look at it. This is called theinstantaneous speed. Instantaneous speed is alsomeasured with a laser or radar speed gun. Theinstantaneous speed of a car changes a lot in theone journey.

At the end of your journey in the car you couldfind the average speed of the car. Average speed isthe speed at which the car would have travelledif it went at the same speed all the time. To findthe average speed we need to know the total distance travelled and the total time taken.Average speed is found mathematically.

Average speed = distance travelledtime taken

There is also average velocity and instan-taneous velocity, which is like average andinstantaneous speed but taking into considerationdirection too.

Speed in metres per second can be converted tokilometres per hour by multiplying it by 3.6. So 1m/s equals 3.6 km/h and 2.8 m/s equals 10 km/h.

Instantaneous speed can be found using a datalogger or other electronic device. These measurethe change in distance over a very short intervalof time. The instantaneous velocity would beshown electronically.

A policeman measures the speed of cars with a radar speed gun

MOTION 103C

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Distance is how ___ it is between two ______. Distance is measured in ______. Time is a _______ of how long it_____ between two ______. In our ___________ time is best measured in _______.Speed is how ____ an object is ______. Most __________ measure speed in ______ per ______. This is abbreviatedto _/_. Velocity is _____ in a given _________.The speedometer of the ___ shows the _____ of the car at the _______ that you look at it. This is called the_____________ speed. Average _____ is the speed at which the ___ would have _________ if it went at the same_____ all the ____.

QUESTIONS

1 What is the difference between:a speed and velocity?b average speed and instantaneous speed?

2 a Cathy Freeman runs 400 m in 49 s. What is heraverage speed in metres per second?

b The winning yacht in the 1999 Sydney HobartYacht Race sailed 1190 km in 44 hours. What wasits average speed?

c A fast bowler in cricket bowls a ball at 40 m/s.The cricket pitch is 20 m long. How long does ittake for the ball to travel down the pitch?

3 Describe an experiment that you could do to find thespeed of a radio-control car being used in thelaboratory.

4 What units of speed and velocity are used in thelaboratory?

5 Complete the table of the speed of some objects,then rank them from fastest to slowest.

Object Speed (km/h) Speed (m/s)

Tennis ball—served by 206Andy Roddick

Arrow fired from bow 82

Cricket ball—bowled by 41Brett Lee

Space shuttle in orbit 8000

Peregrine falconin dive 340

Cathy Freeman sprinting see Question 2a

Ocean racing yacht see Question 2b

Pioneer 10 12 240spacecraft

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measure this distance and the time the ball is in the air

Finding the average speed of a ball

AIM: To find the average speed of various objectsHow fast can you walk? How fast does a sprinterrun? What is the average speed at which you canthrow a ball? (Stay inside the school grounds foryour measurements.)

In order to find these answers, you will need atrundle wheel or a tape measure, a stopwatch anda calculator.Take a note book into the playground to record the measurements.

To work out someone’s walking speed, measurethe distance in metres between two places in theschool grounds. Write it in your note book.Thenmeasure, in seconds, the time that it takes theperson to walk this distance. Write the time in your

note book. Remember that the average speed isthe distance travelled divided by the time taken.Your units are metres per second (m/s).

Use the same method to find the averagesprinting speed and ball speed. Show the results ina table in your science note book.

104

Ticker timer4.2

When driving a car, you can check your speedand distance by looking at the speedometer andodometer. Finding the speed and distance of asprinter or skateboarder is not as easy.

Speed can be studied if it is broken down intosmall segments of time and distance. A simplemethod of doing this is to measure the time ittakes a person to walk along segments of a path.Their journey can be broken into small units oflength and time.

If this person is walking at a constant speed,then they should travel the same distance in thesame time. In the experiment below, the timetaken for the walker to travel 3 m is always thesame. It should not matter which 3 m they arewalking through.

A ticker timer is a method of measuring distance and time together. A ticker timer consistsof a striker, like in an electric bell. The striker hitscarbon paper at regular intervals, each time leaving a dot on a paper tape. The dots are closetogether if the paper is moving slowly and furtherapart if the paper is moving faster. The dots allowus to ‘see’ changes in speed.

Imagine a car driving along a road droppingwater onto the road. It would leave a series ofdrops on the road. Assume that the drops ofwater occur at regular time intervals. If the car istravelling at a constant speed then the dots will bea constant distance apart. The faster the car goes,the further apart the dots will become. If the distance between dots is increasing with each dot,you know that the car is accelerating.

To find the speed, we need to know the distance between the dots and the time betweenthe dots. Remember that speed is distance dividedby time. If each dot is 2 m apart and a drop fallsevery second, then the speed is 2 m divided by 1second. This is two metres per second (2 m/s).

You can slow and measure the motion ofobjects by using a video camera. The movingobject is videotaped and the tape replayed. Usingthe pause button on the VCR will allow you toanalyse the motion. The picture on the videoshould include a ruler and a clock.

In some discos and dance parties there is aflashing light. The motion of dancers appearsjerky and irregular. If you photographed them

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T AIM: To investigate walking speedArrange for some students, each with a stopwatch,to stand in a line 3 m apart.Their task is to measure the time that it takes a person, walking at a constant speed, to reach them.This time isrecorded in a table for class use. Use the followingheadings in the table: Person’s name, Distancefrom start (m), Time taken in total (s),Time taken to travel last 3 m (s).

Redraw the two column graphs shown belowusing the information you have gathered. What dothese graphs show you?

Allowing for student error in using thestopwatch, does it take the same length of time towalk the same distance? What would happen if thestudents were standing closer together? Whatwould happen to the results if the student walkedfaster and faster?

3 m3 m

3 m

walking at a co

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15 1.5

20 2.0

3 36 69 912 1215 1518 1821 2124 24

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Distance travelled(m)

Distance travelled(m)

5 0.5

10 1.0

MOTION 105C

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Speed can be _______ if it is broken down into _____ segments of ____ and ________.A ticker _____ is a method of measuring ________ and ____ together. The dots are close together if the _____ ismoving ______ and further _____ if the paper is moving ______.To find the _____, we need to know the distance between the ____ and the time between the ____. You can slow and measure the ______ of objects by using a _____ camera.If you took a __________ of a moving object in ________ light, you would see ________ images of it. This iscalled __________ photography.

QUESTIONS

1 What two measurements do you need to make tocalculate speed?

2 What is meant by constant speed?

3 How does a ticker timer allow speed to be compared?

4 How do you measure speed using a ticker timer?

5 How can a video camera and VCR allow us to recordand measure motion?

6 Compare the speeds of the objects that recordedthese traces onto paper tape. Compare objects band c to object a.

you would see multiple images of the movingdancers. This idea can be used to record motion.

A stroboscopic light flashes at regular intervals. If you took a photograph of a movingobject in flashing light, you would see multipleimages of it. This is called multiflash photography.Each image would have occurred at a smallamount of time different to the one before andafter it. This lets us see fast motion in a series ofsteps.

A ticker timer only records motion in onedimension (that is, frontwards and backwards). A

stroboscopic photograph shows movement in twodimensions (that is, up and down and left to right).

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T AIM: To use a ticker timer to recordmotionSet up a ticker timer as shown in the diagram.Connect it to a source of 6 V alternating current.The ticker timer should strike the paper at 50times per second, the same as the cycles in alternating current.

Place some paper tape in the ticker timer. Attachthe other end to a trolley or toy car. Let the trolleyor car move down a ramp. As it accelerates, itmoves faster and the distance between the dotsincreases.The speed of the trolley or car can befound by dividing the distance between the dots bythe time between the dots.

Prepare a trace on a paper strip showingmotion.The car or trolley can be moving atconstant speed, or slowing down or speeding up.ticker timer

to AC power supply50 Hz

bricktrolley

ramp

The multiflash photo shows the motion of a ball at different instances

a 2 dots per second

b 2 dots per second

c 10 dots per second

Graphing motion4.3

Graphs are used by scientists, mathematiciansand other people who use data. Data is informationin numerical (number) form. Graphs are a way ofdisplaying data in an easy to see form.

Motion can be shown in graphs. The mostimportant graphs are distance–time graphs andspeed–time graphs.

A tachograph is a device used in large trucksand coaches that automatically records the speed

and travel time. It draws a graph onto a sheet ofpaper or onto a computer screen.

When the motion of an object has changed wesay there has been an acceleration. Accelerationmeans more than just an object speeding up. Theobject may travel faster, or slower, or changedirection. The distance/time graphs and the velocity/time graphs show acceleration. Velocitytime graphs are the easiest for finding acceleration.

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T AIM: To prepare graphs of motionTo complete this experiment you will need a photocopy of the ticker timer drawings shown atthe back of this book (BLM 4.1). Do not reduce orenlarge when photocopying. Photocopies ontocoloured paper are easier to see on a white pagein your note book.

In this experiment, each pair of dots is onesecond apart.

Part 1—Constant velocity (speed) tickertimer recordingAt a constant speed there is an equal distancebetween the dots.This is shown by the top tickertimer recording.

The other recordings show the distance travelledin different time intervals.These range from the

distance travelled in one second to the distancetravelled in six seconds.

Cut these out and glue them to make a graph ofdistance travelled against the time taken.

Use a ruler to join the tops of each column.What is the shape of this graph?

Part 2—Constant acceleration tickertimer recordingWith acceleration, there is an increasing distancebetween the dots.The object making the dots ismoving faster and faster. With constant accelera-tion, the distance between dots increases by thesame amount each time.This is shown on the photocopy.

The other recordings show the distance travelledin different time intervals.These range from thedistance travelled in one second to the distancetravelled in six seconds.

Cut these out and glue them to make a graph ofdistance travelled against the time taken.

Use a ruler to join the tops of each column.What is the shape of this graph?

Part 1 - Constant velocity ticker timer recording

Part 2 - Constant acceleration ticker timer recording

Part 3 - Finding velocity from ticker timer recording

Part 1 - Constant velocity

ticker timer recording

106

MOTION 107

Part 3—Finding velocity from tickertimer recordingsVelocity (speed) is the distance travelled in eachunit of time. On our ticker timer tape, the timebetween the dots is one second.The tape has been drawn so that the distance between the dots represents the speed of the object.

The top ticker timer tape in Part 3 is similar tothat in Part 1.This tape has been divided into timesections of one second each. Cut out each sectionand make a velocity–time graph.The length of eachsection of tape represents velocity.

Use a ruler to join the tops of each column.What is the shape of this graph?

The bottom ticker timer tape in Part 3 is similarto that in Part 2. It was made by an objectundergoing constant acceleration.This tape hasbeen divided into time sections of one secondeach. Cut out each section and make avelocity–time graph.The length of each section of tape represents velocity.

Use a ruler to join the tops of each column.What is the shape of this graph?

You have drawn four line graphs in thisexperiment. As a summary, draw a simple linegraph of the following graphs.

■ distance–time graphs for constant speed■ velocity (speed)–time graph for constant speed■ distance–time graphs for constant acceleration■ velocity (speed)–time graph for constant

acceleration

dist

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Motion can be shown in ______. The most important graphs are ________–time graphs and _____–time graphs.A tachograph is a device used in large ______ and _______ that automatically records the _____ and travel ____. Itdraws a _____ onto a sheet of _____ or onto a ________ screen.

QUESTIONS

1 What are the two main types of graphs showingmotion?

2 How can the record of a ticker timer be convertedinto a motion graph?

3 What is acceleration?

4 Tachographs are fitted to commercial trucks andbuses. What can the operators and road safetypeople learn from the tachograph in a bus?

5 The graph shown on the right is a distance–timegraph. It shows the distance away from home of agirl during the day. She went to school, to the shopsand to sports training during the day. Answer thesequestions from the graph.

a How far is it from the girl’s home to school?b When did the girl leave school?c The shops are 500 m away from home. When did

she leave and return home?d What time is sports training? How long does it

last for? When does the girl return home?

Time of day

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8 9 10 11 12 1 2 3 4 5 6 70

0.5

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Part 2 - Constant acceleration

ticker timer recording

Other motion4.4

The motion we have studied so far is motion in astraight line. Real motion is more complex thanthis. Some other types of motion are studied inthis activity.

Circular motion is the movement of an objectin a circle. Examples include a spinning toy,twirling an object over your head, a record or CD,a satellite orbiting the Earth, and the Earth orbiting the sun.

Speed in a circle can be measured in two ways.One is metres per second, like speed in a straightline. This is the speed of a satellite or a speck ofdust on the outside of a record. For example, theEarth moves around the sun at 29 km per secondand the space shuttle moves at 8 km per second.

Speed in a circle is best measured in revolutionsper minute, or rotations per second, or someother measure of how many circles are made in aunit of time. For example, the Earth orbits thesun once per year, a satellite orbits the Earth onceevery 90 minutes, and a ball (attached to string)rotating over your head may complete a circleevery two seconds.

A pendulum is an object that swings to and froon the end of a string or bar. Pendulums were

once used to keep time in clocks, because thelength of time for each swing was the same. Themovement of something to and fro is called harmonic motion.

The length of time taken for one swing of apendulum depends on several factors. These areinvestigated in the experiment on page 109.

An example of harmonic motion is an object

clothes drier

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T AIM: To investigate circular motionPut a tennis ball into an old sock or stocking, andtie it onto a length of string.Twirl the ball aroundand around over your head. Control the speed sothat the ball just stays horizontal as it goes roundand round. Measure the time taken for ten swings.Record this time and other details in the table(right). Repeat the procedure with different lengthsof string.

Length of Time for Time for string (m) 10 orbits (s) 1 orbit (s)

1.01.52.02.53.0

Write a sentence explaining the relationship (theconnection) between the length of string and thetime taken for one orbit. Write this experimentcorrectly in your note book, with an aim, method,diagram in pencil, table of results, and a conclusionthat relates to the aim.

Repeat the experiment to find the effect on theorbit time of changing the mass of the ball. Do thisby adding a second and even a third ball. Keep thelength of string constant.

Write a sentence explaining the relationshipbetween the orbiting mass and the time taken forone orbit.

Circular motion

Swing the ball around your head while another student records the time

108

MOTION 109

bouncing up and down on a light spring. Motionwill continue until it is slowed by friction (in thespring and with the air).

Cars have springs to smooth the ride whenthey drive over bumps and holes in the road. The

springs are ‘dampened’ (or vibrations arestopped) by shock absorbers.

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T AIM: To investigate a pendulum swingSet up a weight (sometimes called a ‘bob’) on theend of a light string. Set the weight swinging, withjust a small sideways movement.

Using a stopwatch, measure the time taken forten complete swings. Divide this time by ten to findthe average time taken for one complete swing.

What affects the time for one swing? Conduct aseries of experiments to find the effect of changingeach of these:

■ distance of one sideways swing■ mass of the bob■ the length of the stringUse metres, kilograms and seconds for your

measurements. Record your results in your sciencenote book.

Write this experiment correctly in your notebook, with an aim, method, diagram in pencil, tableof results, and a conclusion that relates to the aim.

tie string to clamp

10 g or 20 g bob

arc of swing

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Circular motion is the ________ of an object in a ______. _____ in a circle can be measured in ___ ways. One is ______ per second, like speed in a ________ line. Speed in a______ is best measured in ___________ per minute, or _________ per second.A ________ is an object that swings __ and ___ on the end of a ______ or bar. The ________ of something to andfro is called ________ motion.

QUESTIONS

1 What is meant by circular motion and harmonicmotion?

2 Galileo (so the story goes) was sitting in churchwatching a chandelier swing. It had been setswinging by the person who had lit the candles in it.Galileo used his pulse to measure the time taken foreach swing.a Why did Galileo use his pulse?b What results would Galileo have found?c Why are pendulums used in some clocks?

3 If you were building a pendulum clock, how wouldyou experiment to find the length of a pendulumthat would give a complete swing of one secondexactly?

4 The speed of a car engine is often quoted in rpm.What does rpm stand for?

5 What is the speed, in revolutions per minute, of thesecond hand and minute hand on an analogue clock?

6 Cyclists race on a track called a velodrome. It issemi-circular at each end and straight on the sides.

a Describe the motion of a cyclist completing one lap.

b The illustration shows a cyclist on the velodrome track. How is friction reduced by the cyclist?

7 If you were a space engineer designing a communications satellite, what variables would you need to consider in order to keep the satellite at the correct height and speed?Communications satellites stay above the same part of the Earth.

straight track

straight track

semi-circulartrack

start/finish

Velodrome

Swinging the pendulum

110

Gravity and acceleration4.5

If you step off a tall building you will fall. Gravityexerts a force on your body and pulls you towardsthe ground. The further you fall, the faster youfall. This is because you are being accelerated asyou fall. Large people and small people accelerateat the same rate.

Gravity causes acceleration on all objects. Theyare accelerated towards the centre of the Earth.The size of the acceleration due to gravity can befound by experiment. You can use a pendulum or

an electronic measuringdevice that measures afalling ball.

There is a speed limitto falling. As you fallthrough the air, the airpushes back againstyou. This is air resistance,and it is a type of friction.The highest speed youcan reach is called terminal velocity. Itdepends on the amountof air resistance you

have in your fall. Parachutists in free fall reach200 km/h (60 m/s). They slow considerably whenthey open their parachutes. A parachute providesmuch more air resistance.

There is a story that the famous scientist,Galileo Galilei, in the 1500s, dropped a large ironcannon ball and a small iron ball from theLeaning Tower of Pisa. He wanted to know if theyhit the ground at the same time. People suspectedthat the larger cannon ball would hit the groundfirst because it was heavier. However theobservers saw that both balls hit the ground at thesame time.

When the NASA astronauts were on the moon,they did a similar experiment. They dropped afeather and a hammer, and filmed it with a moviecamera. They, like Galileo, wanted to know if theacceleration was the same for different objects.There is no atmosphere on the moon to slowfalling objects with air resistance like there is onEarth. The feather and hammer hit the surface ofthe moon at the same time.

Gravity on the moon is much weaker thangravity on Earth. On the moon you can jump

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T AIM: To investigate gravityA pendulum keeps moving to and fro because ofthe energy it has. Gravity keeps pulling the bobback towards the middle because this is the lowestpoint.The size of the acceleration due to gravitycan be found by the equation

g = 4π2

T 2

Make a pendulum about1 m long and secure it to aclamp. Swing it about 10 cmside to side. Measure thetime to make ten completeswings, and divide this byten to find the time forone swing (T). Substitutethe value for T into theequation, and use a calculator to find g. Check withyour teacher for the real value of g. How correctare you? Why might your result be inaccurate?

Alternative methodYour teacher will use an electronic method to measure the time taken for a ball to fall a knowndistance. Using the equation g = 2h

t2

(where t = time taken and h = height the ball falls from) you can calculate g, the size of theacceleration of gravity. Repeat the experiment fordifferent heights, to see if you get the same valuefor g each time.

l = length of string and bob

T= average time for one complete swing, to and fro. 00 0 0 0

electronicclock

rulerelectromagnet

height,symbol h

steel balllight sensor—clock starts when the light beam is cut by the falling ball

light sensor—clock stops when the light beam is cut by the falling ball

How the electronic timer is set up

Parachutes provide air resistance

MOTION 111

much higher than on Earth. If you jumped off acliff on the moon you would take much longer toreach the ground than on Earth. Again, this isbecause gravity on the moon is not as strong ason Earth.

On the moon objects fall with an accelerationdue to gravity of 1.6 m s–2. We say this as 1.6metres per second squared. It means 1.6 metresper second per second. A falling object changes itsspeed by 1.6 metres per second every second.Since there is no atmosphere on the moon to produce air resistance, falling objects fall fasterand faster. Their speed at the end of the first second is 1.6 m/s, then 3.2 m/s at the end of thenext second, then 4.8 m/s at the end of the thirdsecond, and so on. The change in speed is 1.6metres per second every second, or 1.6 metres persecond per second.

Mathematically, acceleration is the change inspeed divided by the change in time. It can bewritten as:

Acceleration = change in speedchange in timee

If a motorbike accelerates from 2 m/s to 6 m/sin 4 s, then its change in speed is 6 – 2 = 4 m/s.Its acceleration is change in speed (= 4 m/s) divided by the change in time (= 4 s). This equals1 m/s per second, or 1 m/s2, or 1 ms–2.

Examples of objects accelerating are a car moving off from the traffic lights, a rocket beinglaunched, a sprinter starting a race and a jet airliner at take-off. Deceleration is slowing down,or negative acceleration. Acceleration is also achange in direction. In circular motion and harmonic motion the objects are constantly beingaccelerated.

A force produces acceleration. A force is a pushor a pull that changes motion. Acceleration is ameasure of how fast motion changes.

Our weight is caused by gravity. The gravitational acceleration pulls us down onto theEarth. The acceleration and our mass cause ourweight. Our weight is the downward force weexert on the floor when we stand and walk. Onthe moon there is less gravity, so we would haveless weight. In space where there is no gravity wewould have no weight. This is called weightlessness.

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Gravity causes ____________ on all objects._______ on the moon is much ______ than _______ on Earth. On the ____ you can ____ much ______ than onEarth.On the moon objects ____ with an ____________ due to _______ of 1.6 m s–2. A falling ______ changes its _____by 1.6 metres per ______ every ______.Deceleration is _______ down, or ________ acceleration. Acceleration is also a ______ in direction.A _____ produces ____________. A force is a ____ or a ____ that changes ______. Acceleration is a ______ of howfast ______ changes.Our ______ is caused by _______. The gravitational ____________ pulls us down onto the _____.

QUESTIONS

1 What is acceleration? Objects can have the samespeed and be accelerated. When does this happen?

2 What is the difference between acceleration anddeceleration?

3 Describe the acceleration or deceleration actingwhen:a you jump from an aeroplane

b a car slows and stops at the traffic lightsc a satellite orbits the Earth.

4 What is weightlessness? What causes it?

5 Compare the heights and distances that could bereached by athletes competing on the moon and onEarth. Give reasons.

6 Acceleration is measured in m/s2. Use the meaningof acceleration to explain how this unit is derived.

NASANASA

NASA’s acceleration experiment

Review and Research

112

Measuring motionReview questions

1 Match the word with its meaning. Select fromthese words. acceleration, circular, data, graph,harmonic, pendulum, speed, ticker timer,velocity, weighta how fast something is movingb going faster, slower or changing directionc information in numerical formd visual way of displaying datae leaves a dot on a paper tape to record motionf speed in a given directiong motion of an object going round and roundh motion of an object going to and froi swinging object that can be used to

measure time j downwards force due to gravitational

acceleration

2 Are these sentences true or false? If they arefalse, state why they are incorrect.a A heavy stone falls faster than a light stone.b On the moon you can jump higher because

the gravity is less than on Earth.c Gravity causes objects to accelerate towards

the Earth.d Near the Earth, air slows falling objects.e If an object has a constant speed, then it

takes the same time to travel the samedistance.

f When there is no air, gravity is stronger andobjects accelerate faster.

g As objects travel at a higher speed, theytake less time to travel the same distance.

h If you jump from an aeroplane you will fallfaster and faster the further you fall.

i An acceleration of 2 m s–2 means that speedincreases by 2 metres every second squared.

j Deceleration is more powerful acceleration.

3 In 1998 the champion racehorse Might andPower covered 800 m in 38 seconds in apractice gallop. What is the average speed inmetres per second? Multiply this by 3.6 to findthe average speed in kilometres per hour.

4 A tractor is moving along a straight flat road,and oil drips onto the ground at regularintervals. The figure (top right) shows the dropson the ground when travelling at 20 km/h.

How is the tractor moving in each of the following situations?

a

b

c

d

5 A car travels 120 km in 2 hours. During thistime it reaches a maximum speed of 85 km/hon a freeway, and stops for 15 sets of trafficlights. What is the maximum speed, theminimum speed, and the average speed of the car? Include units.

6 A ticker timer leaves 10 dots per second. Thetime between each dot is s, or 0.1 s. If anobject travels 5 cm in 0.1 s, what is its speed?

7 Study the distance–time graph drawn below.

a What is the greatest distance reached?b How long did it take to reach this distance?c Between which times was the object

moving?d What was the motion of the object from

t = 8 s to t = 12 s?

8 Study the speed–time graph drawn below.

110

Time (s)

Dis

tanc

e (m

)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

123456789

10

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Spee

d (m

/s)

0 10

10

20

20

30

30

40

40

50 60

Oil drops when the tractor travels at 20 km/h

a What is the speed after 20 seconds?b When did the object reach 20 m/s?c What was the initial speed of the object?d Was the speed uniform? How do you know?

9 A tachograph records the speed and travel timeof large vehicles. This data is shown as avelocity–time graph. Answer the questionsbelow about the tachograph.

The tachograph started recording when thedoors closed.a How long was it from when the doors closed

to when the bus moved away?b When did the bus stop again?c What was the highest speed the bus

reached?d When did the bus complete its journey?

10 The sign shows a police speed check warningsign.

How could you measure the speed of a car ona highway from an aircraft?

11 The data below shows the speed of a skydiveron Earth and the speed the skydiver wouldhave on the moon. (The data has beencalculated, not measured by experiment.)a Graph the data as a line graph, with speed

on the y-axis and time on the x-axis. Labelthe axes and show the units.

b Explain why the skydiver on Earth reaches aterminal velocity.

c Why does a skydiver need a parachute?d Why is it not advisable to go skydiving and

parachuting on the moon?

Research questions

1 The picture shows agyroscope.a What are gyroscopes

used for? b Why do bicycles fall

over when they are stillbut tend to stay uprightwhen they are moving?

2 Gravity is an important force in astronomy. Ithelps form galaxies, solar systems and blackholes. How strong is gravity in the universe? Howdoes gravity control our solar system and galaxy?

Extension experiment

AIM: To measure speed with a ticker timer

1 Calibrating the ticker timer. You can check theticker timer, or calibrate it, by counting thedots it leaves per second. To do this, pull apaper tape through the ticker timer. Use a stopwatch to measure how long it was turnedon for. Count the dots left on the paper, anddivide by the number of seconds. This will tellyou how many dots per second the ticker timermade. Do not adjust the ticker timer or thevoltage supplied until after you have recordedall the tapes you need.

2 Use sticky tape to attach some paper tape to atrolley. Pull the trolley and paper tape throughthe ticker timer. Select a section of tape withclear dots on the paper.In calculations, you need to know the distance

between the dots and the time interval betweenthe dots. Measure the distance with a ruler. Ifthere were 43 dots per second, then there was

of a second, or 0.023 s, between each dot.Speed is found by using distance/time. If you

recorded distance in centimetres, and time inseconds, then speed is in cm/s.

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Time (min)

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city

(km

/h)

0 1 2 3 4 5 6 7 8 9

10

20

30

40

50

time (s) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15speed on

0 9.8 19.5 29.2 38.8 47.9 53.5 57.0 57.0 57.0 57.0 57.0 57.0 57.0 57.0 57.0Earth (m/s)speed on

0 1.6 3.2 4.8 6.4 8.0 9.6 11.2 12.8 14.4 16.0 17.6 19.2 20.8 22.4 24.0moon (m/s)Speed of a skydiver on Earth and the moon

Gyroscope

MOTION 113

rope Pull force

A pull force

114

Pushes, pulls and twists4.6

A force is a push or pull or twist. A force canchange the motion of an object. The motion maybe faster, slower or in a different direction. Aforce can also change the shape of an object, forexample when you squash a balloon betweenyour hands.

When you kick a soccer ball your foot pushesthe ball. Your foot exerts a force on the ball,which causes its motion to change. This type offorce, where you have touched the ball, is calleda contact force. Some other contact forces arepushing a door, hammering a nail and lifting abook. Every time you touch and move an objectyou have exerted a contact force. Pushes andpulls are the most common contact forces.

A contact force that you might not think aboutis called weight force. When you are standing orsitting you are exerting a weight force down-wards. This force is caused by gravity. You can seethe weight force when you stand on foam rubberor on a plank of wood.

Force is measured in newtons. The namecomes from Isaac Newton, who first explainedthe ideas of force, gravity and weight. The abbreviation for newton is N. The force of youpushing against the wall is about 200 N. A onekilogram book has a weight force of 10 N.

Some forces work without hitting or touching.When you step out of an aeroplane a force pullsyou downwards to the Earth. There are no contact forces touching you. Gravity is a forcethat acts without touching you. Gravity andforces like it are said to ‘act at a distance’.

Electrical and magnetic forces act at a distance.You can see these forces with magnets andcharged rods. There is a magnetic field around themagnet, and an electric field around the chargedrod.

The force of gravity is due to the gravitationalfield around the Earth. The gravitational fieldpasses through buildings. It pulls us towards theEarth.

There are other forces that are hidden. Theseforces do not cause movement but stop movement.

When you stand on a plank, you exert a weightforce. The plank may change shape and bend.Why do you stop moving down? The plank pushes upwards, and the upward push of theplank equals the downward push of your weightforce. This upward force is called the reaction

Push force

A push force

magnet

paper clip

cotton thread

sticky tape

This force is due to a magnetic field

plastic

paper pieces

This force is due to an electric field

the weight force was greater than the reaction force

weightforce

reaction force

The forces involved with people on a plank

MOTION 115C

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INT: COPY AND COMPLETE

A force is a ____ or ____ or _____. A force can change the ______ of an object. A force can also ______ the _____of an ______.Every time you _____ and ____ an object you have exerted a _______ force. Pushes and pulls are the most ______contact ______.When you are ________ or _______ you are exerting a ______ force downwards.Force is measured in _______. The ____________ for newton is _.Gravity and forces like it are said to ‘___ __ _ ________’. A reaction _____ pushes _______ your force. The ________ force stops you from ________ through the floor.The force that _____ movement is called ________. Friction is the _____ that pushes _______ ______ objects.

QUESTIONS

1 What is a contact force? Give two examples.

2 What is a weight force?

3 Name three forces that act at a distance. What isneeded for these forces to act?

4 What is a reaction force? Why are reaction forcesimportant?

5 What is the force of friction?

6 What is a force field? Which force field acts on yourbody to cause your weight?

7 Ice skaters move across the ice without much effort.How do they move so easily?

8 Describe what would happen if you tried to ride yourbike in a world without friction.

force. If two people stand on the plank it bendsmore and pushes upwards more.

When a plank cannot push upwards any moreit breaks. A steel beam has a stronger reactionforce than a wooden plank, and it will not break.

A reaction force pushes against your force.Reaction forces are there when you are standingon the floor, or when you are pushing against thewall. The reaction force stops you from crashingthrough the floor or pushing the wall over.

The force that slows movement is called friction. It is best called the force of friction or frictional force. Friction is the force that pushesagainst moving objects. A ball rolling across anoval, a bike or skateboard rolling along the roadall slow and stop because of friction.

Sky divers fall throughthe air. The air pushesback to slow their fall.The air provides frictionas they fall.

Cars push through the air as they drive along.Cars are made smooth, or streamlined, to reducefriction with air. Tests are done with cars andsmoke in a wind tunnel.

We are accustomed to friction. Living withoutfriction would be very strange. An ice rink has alow friction surface. It is hard to walk or stand onthe ice.

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T AIM: To make a puckA puck is a small toy that moves without much friction.To make one you will need a flat piece ofwood or metal about as large as your hand. Drill a hole through the middle of the sheet.The sheet mustbe smooth on the bottom. Attach a tube or pipe overthe hole.

Blow up a balloon and stretch it over the tube. Asthe air leaves the balloon it creates a layer of airunderneath the sheet of wood (or metal) that makes a low friction surface, allowing the puck to move.

balloon

wood or thin metal with

holesmooth bottom

tube – attach to base over hole

The air provides friction as a skydiver falls

116

Mass and weight4.7

At the butcher’s shop you would ask for two kilograms of sausages, and your science teachermay ask you to measure out ten grams of coppersulfate. The butcher measures the weight of thesausages, while your science teacher wants you tomeasure the mass of copper sulfate. What is thedifference between mass and weight? Is it important?

The mass is the amount of substance, oramount of matter, which is inside an object orperson. Mass is measured in kilograms, abbreviatedto kg. Small masses are measured in grams, g.Weight is a downward force, which depends onyour body’s mass and the strength of gravity.Weight, like all forces, is measured in newton,symbol N.

The butcher weighed 2 kg weight of sausages.But weight is measured in newton. What is goingon?

In everyday life, sausages can be measured inkilograms mass or kilograms weight. You wouldget the same amount of sausages. When you arebuying sausages the difference is not important.But to a person measuring forces the difference isimportant.

Mathematically,

Weight = mass × acceleration due to gravityor in symbols W = m × g

On Earth, the strength of the acceleration dueto gravity is nearly 10 metres per second per second, written as 10 m s–2. This means that anobject falling in the Earth’s gravity will go fasterby ten metres per second for every second whileit is falling.

Weight is important in cases where forces areimportant. Consider this person:

■ 70 kg mass■ 700 N weight (70 kg x 10 m s–2)For most people, mass and weight are similar

and measured in kilograms. Weight force in newtons is important for scientists. Some examples of forces involving weight are on rollercoaster rides, space flights, aerobatic aircraft andriding in a lift.

If you stand on some bathroom scales whenyou are riding in a lift in a tall building you willsee your weight change.

When the lift accelerates upward, the floor ofthe lift pushes upwards. Your feet push harderagainst the floor and bathroom scales. This extraforce downwards is extra weight. The scaleswould show that you are heavier than normal.When the lift is accelerating upwards you feelheavier. Your stomach feels like it is going downinto your feet. This only applies when the lift isaccelerating upwards.

The opposite happens when the lift slows. Youfeel lighter. Your stomach feels like it is rising upinto your throat.

When the lift accelerates down, the lift floor istending to move from under your feet. Your feetdon’t push as hard against the floor or against thebathroom scales. The reading on the scales is less,and so is your weight.

Part of the fun of a rollercoaster ride is experiencing the forces that act on your body.One of these is the weight force. A rider accelerating down a 60° slope will only feel 50%of their weight. As the track levels out at the bottom of a 45 m drop, the riders will weigh threetimes their normal weight. As the rollercoaster

When the lift accelerates upward you press harder against the floor

You get heavier when riding up in a lift

When the lift accelerates downyou do not pressas hard againstthe floor

When riding down in a lift you get lighter

MOTION 117

COPY AND COMPLETE

The mass is the amount of _________, or amount of ______, which is ______ an ______ or ______. Mass ismeasured in _________, abbreviated to __. Weight is a ________ force, which _______ on your body’s ____ and thestrength of _______. Weight is measured in ______, symbol N.When the lift ___________ upward, the _____ of the lift pushes _______. Your feet push ______ against the _____and ________ scales. This extra force _________ is extra ______.The ________ happens when the lift _____. You feel _______. Astronauts experience _______ in their ______ as they leave Earth, travel in _____ and land on a __________ ormoon. This is called ___________ and only occurs when there is no ___________ .

QUESTIONS

1 What is the meaning of mass and weight?

2 Why is weight a type of force?

3 What units are used to measure weight, mass andforce?

4 How does your weight change on a rollercoaster ride?

5 Why do you feel heavier when a lift in a tall buildingstarts moving upwards?

6 When you stand on the bathroom scales, the topsinks downwards and then pushes upwards to balanceyour weight. What type of force is the upward force?

7 Describe why the weight of the astronaut changes inthe diagram above.

8 Assume that you could travel and land on a neutronstar, where the gravity is ten million times greaterthan Earth. What would be your weight? What wouldbe your mass? What would happen to your body?

hurtles over the top of a rise, the forces change sothat the riders weigh almost nothing.

Rollercoaster tracks are very strong. They arebuilt to support the forces of the rollercoaster trolleys and their human riders as their weightchanges during the ride.

Pilots in aerobatic aircraft and military jet fighters experience large forces on their bodies incertain manoeuvres. When flying in tight circles,the acceleration produces a large force, or weight,on their bodies and the seat they sit on.

Astronauts experience changes in their weightas they leave Earth, travel in space and land on aplanet or moon. An astronaut whose mass is 70 kg would weigh nothing in space, 0 N and 0 kgforce. This is called weightlessness and onlyoccurs when there is no gravity.

If this 70 kg astronaut landed on a planet withgravity 20 m s–2 then their weight force is 1400 N

(70 kg x 20 m s–2) or 140 kg force on their bathroom scales.

on Earth

at lift-offin spacein space

on the moonon the moon

W = 700 N

850 N

0 N0 N

115 N

The astronaut’s weight changes on this space trip

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kg m

ass

N fo

rce

kg fo

rce

50

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use on Earthgravity = 10 m s-2

kg m

ass

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rce

kg fo

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100

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120070

1400140

80

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Bathroomscales for use

on a planet where gravity = 20 m s-2

Weight varies with the forceof gravity

Pilots in aerobatic aircraft experience large forces on their bodies

118

Inertia4.8

When a car or bike crashes it stops quickly. Butthe driver or rider keeps moving. Their injuriesoccur when they crash into the windscreen ortravel over the handlebars and hit the ground.They are following one of the rules of motion.

Inertia is the tendency of an object to stay stillor to keep moving. You feel it when you arestanding in a bus. When the bus starts moving the

floor moves, but your body stays still. As a resultyou move backwards. When the bus stops, yourbody keeps moving forwards.

When a car crashes it stops suddenly. The driver and passengers have inertia, and they keepmoving. If they are not restrained by seat beltsthey can crash into the windscreen or go through it.

These are examples of inertia. Objects that areat rest will stay that way until something makesthem move. Objects that are moving will keepmoving with the same speed in the same direction until something changes their motion.

The concept of inertia is very important in oureveryday lives. We notice it in cars and buses asthey speed up or slow down. Inertia causes us toslide across the seat when buses and cars goaround a corner too quickly.

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When the bike suddenly stops, the rider keeps moving

AIM: To investigate inertiaPart 1The effects of inertia can be studied with a toy carand a plasticine figure that rides on the roof of thecar. Roll the car down a 30 cm ramp and crash thecar into a book.The plasticine figure should keepmoving because of its inertia. It is difficult to recordmeasurements in this experiment so comparisonsare made instead.

Write the heading Experiment—Inertia into yournote book. Read the following steps for thisexperiment and write an aim into your book. Drawa diagram (with a ruler) of the procedure.

1 What is the effect of speed? Crash the toy carinto a book, using low speed and medium speed.How does the plasticine figure move? Whendoes it have the most inertia?

2 What is the effect of mass? Roll the toy carfrom the same place on the ramp, using a largefigure and a small figure. Which plasticine figure

travels the greater distance? Which figure exertsthe greater force when it moves forward?What factors affect the inertia of a person in a car?Write two sentences as your results of Part 1.

Part 2Modern cars are fitted with seat belts and energy-absorbing crumple zones.1 What is the effect of wearing a seat belt? Tape

the plasticine figure onto the car, as if it waswearing a seat belt.

2 What is the effect of energy-absorbing crumplezones? Tape some foam rubber across the frontof the toy car. Crash it into a book and comparethe inertia of the plasticine figure.

What factors could reduce the effects of inertiaon a passenger in a car? Write two sentences asyour results of Part 2.

Write a conclusion for the whole experimentstating what factors have an effect on inertia, andhow the inertia can be reduced in car crashes.

toy car

plasticine figure

piece of paper between figure and car

foam rubber

The experimental set-up

Investigating a crumple zone

A seismograph records earthquakes. It consistsof a large heavy object held by a spring. When anearthquake strikes, the ground moves. The seismo-graph stays still because of its inertia. A recordingpen shows the movement of the Earth relative tothe seismograph.

In 1972 the space probe Pioneer 10 waslaunched to explore the outer planets. It is thefirst human-made object to leave our solar system. Travelling at 12.24 kilometres per second,Pioneer 10 is expected to travel at the same speedand in the same direction for two million years.Eventually a force will change its motion. Theforce might come from the gravity of the distantstar Aldebaran or possibly even an extraterrestrialinvestigating the space probe.

On Earth we are surrounded by forces. Themost common forces are the gravitational force

and friction. Friction is the force that slows moving objects. It is sometimes called frictionalforce. In a bike the friction comes from the tyreson the road, the axle and bearings, and frictionwith the air. This is called air resistance.Parachutists use the friction of air resistance toslow their motion.

Most of the injuries in road crashes happenwhen occupants hit the dashboard or windscreen,or are thrown through the windscreen. The occupants of cars have inertia and they keepmoving after the car has stopped.

The ideas of motion, including inertia, werefirst described by Galileo and later by IsaacNewton in the 1500s and 1600s. The concept ofinertia is described in Newton’s first law ofmotion. It says: ‘A body remains at rest, or continuesto move in the same direction with the same speed,unless acted upon by a force.’ This describes themotion of Pioneer 10, the passengers in a bus anda plasticine figure on a toy car.

MOTION 119C

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INT: COPY AND COMPLETE

Inertia is the tendency of an ______ to stay _____ or to keep ______.The concept of _______ is very _________ in our ________ lives. We notice it in cars and buses as they speed up orslow down. Inertia causes us to _____ across the ____ when _____ and ____ go around a ______ too quickly.A seismograph records ___________. It consists of a _____ _____ object held by a ______. The ___________ staysstill because of its _______.Pioneer 10 is expected to ______ at the same _____ and in the same _________ for two _______ years. Eventuallya _____ will change its ______.Friction is the _____ that slows ______ objects. It is sometimes called __________ force.The concept of _______ is described in ______’s _____ law of ______.

QUESTIONS

1 What, in your own words, is Newton’s first law ofmotion?

2 In what ways does Pioneer 10 show inertia?

3 How does a seismograph work?

4 How does inertia contribute to injuries in carcrashes?

5 Why do seat belts reduce injuries in car crashes?

6 What frictional forces slow a pushbike?

7 Why does friction change the inertia of objects?

8 Can inertia apply to stationary objects? Explain withreasons.

9 The illustration above shows a car travelling around acorner. What would happen if the car lost friction?

A seismograph uses inertia to measure the Earth’s movement

What would happen if this car lost friction?

ARTWORK TO COMESS45.32B

120

Force and acceleration4.9

A force is a push, or pull, or twist that changesmotion. The size of a force is measured in newtons(symbol N). Some common measures of force are:■ switching a light switch, 5 N■ removing a ring tab from a can, 20 N■ hitting a tennis ball with a racquet, 2000 N■ the force of a jet engine, 200 000 N

When you are riding in a car, you are being accelerated all the time. Moving away from traffic

lights, going around a corner and stopping allinvolve acceleration. Remember that accelerationis a change in speed or direction. Deceleration isslowing down, a negative acceleration.

A force is a push or a pull that causes a change inmotion. A change in motion is acceleration. Tochange the acceleration a force is needed. In a car,force comes from the engine, the brakes and thetyres. The force exerted by the engine will accelerate the car from rest. The force exerted bythe brakes will cause the car to stop. The forceexerted by the tyres will cause the car to changedirection.

The force exerted by a cyclist to move a bike isnot large enough to move a car. The force exerted by a car engine is hardly large enough tomove a truck. The experiment below is aboutinvestigating the relationship (connection)between force, acceleration and mass.

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T AIM: To investigate the relationshipbetween force, acceleration and massFor this experiment, you can use either a collisiontrolley or a toy car, string and masses.

If you are using a collision trolley, the force isapplied by a spring under the trolley. Hold thetrolley against a wall or solid barrier, and releasethe spring.To estimate the acceleration, measurethe distance that the trolley moves. Do this threetimes and take an average.

Repeat for different forces and different masseson the trolley. How can you make the accelerationof the trolley greater? What reduces theacceleration?

If you are using a toy car, string and masses, thegravitational force of the masses will move the toy

car. (Since toy cars have such a small mass, it is bestto use small forces to move them.Twenty-centcoins taped onto string work well, as do metalwashers tied onto string.) Estimate the accelerationof the car from its speed across the table.

Repeat for different forces and different masses onthe toy car. How can you make the acceleration ofthe toy car greater? What reduces the acceleration?Consider the force and mass of the car.

Write your results in a table. Suitable columnswould be Size of force, Mass of car or trolley and Sizeof acceleration. Combine your results and makesome conclusions about the experiment. Writeyour conclusion in your note book.

different tensions in spring= different forces

place books on trolley to increase the mass

Mass on string. Start with about 20 g mass. Different masses give different forces

Add other cars or plasticine to change the massEstimating acceleration

Investigating how mass and force affects acceleration

force from a jet engine

force to open a drink canforce to switch on a light

force on atennis ball 200 000 N

20 N5 N

2000 N

Measures of force

MOTION 121C

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INT:

QUESTIONS

1 What, in your own words, is Newton’s second law ofmotion?

2 What two factors determine the acceleration of a carfrom rest?

3 Imagine two similar cars, one of which is towing acaravan. The mass of the caravan is equal to themass of the car. Compare the car that is on its ownto the car towing the caravan in these situations.a What is the difference in acceleration?b What is the difference in stopping force when the

brakes are applied?c What is the difference in decelerating force when

the brakes are applied?

d Which car takes the longer distance to stop?Explain clearly.

4 The modern unit of force is called the Newton. Whodoes this name honour? What did he do?

5 How would the acceleration of a car change if a truckmotor replaced the usual motor? Consider the changein force and mass.

6 Which accelerates faster: a 400 000 kg Boeing 747jet whose engines exert a total force of 800 000 N, asports car that exerts a force of 7800 N with a massof 1300 kg, or a motorbike of mass 300 kg with anengine force of 3000 N?

COPY AND COMPLETE

When you are riding in a car you are being ___________ all the time. Remember that acceleration is a ______ in_____ or _________. Deceleration is _______ down, a ________ acceleration.A force is a ____ or a ____ that causes a ______ in motion. A change in motion is ____________. To ______ theacceleration a _____ is needed. In a car, force comes from the ______, the ______ and the _____.The ____________ caused by a _____ is increased if the _____ is increased. The ____________ caused by the same_____ is reduced if the ____ is increased. This is usually written as the ____________ equation _ = __.

In the experiment you would have noticed thata big force produces a big acceleration. But thesize of the acceleration also depends on the mass.A big mass reduces the acceleration.

We can understand this with some commonexamples. A car with a powerful engine willaccelerate faster than a car with a less powerfulengine. Powerful engines give more force andmore acceleration.

The acceleration of the car also depends on themass of the car. A car loaded with heavy luggagefor holidays has more mass, and will not have thesame acceleration as when it is unloaded.

The loaded car will not stop as well as anunloaded car. The braking force in cars is providedby the brakes. The car with the greater mass will

take longer to stop. Again the mass reduces thedeceleration. The mass is like inertia. It resists thechange in motion caused by the force.

The acceleration caused by a force is increasedif the force is increased. The acceleration causedby the same force is reduced if the mass isincreased. Mathematically,

a = Fm

This is usually written as the mathematicalequation F = ma. Force is measured in the unitcalled Newton, mass is measured in kilograms,and acceleration is measured in metres per second squared. The abbreviations are N, kg andm s–2.

Isaac Newton summarised this information inhis second law of motion. It says that: ‘A large forceproduces a large acceleration, provided that the massremains the same. A large mass reduces the accelerationof the same force.’

In a car crash the occupants stop moving quickly. There is a large deceleration. A largeforce is needed for this to happen. A large forcecan be enough to break bones and cause internalinjuries. Air bags, padded dashboards and crumple zones in cars absorb a lot of the forces incollisions, and reduce the forces applied to occupants as they suddenly stop.

Ocean going ships have a large mass. In spite of huge engines generating a large force, the acceleration is small

Reaction forces are all around us. When you standon the floor, you exert a weight force downwards.Yet you do not go crashing through the floor. Thefloor pushes upwards against the weight force.This invisible upward force is called the reactionforce.

If you run quickly on a smooth floor you mayslip over. The floor doesn’t provide the frictionforce needed to stop your feet from slipping.When your shoes push on the floor, the floorpushes back. The force you provide is called theaction force. The force the floor uses to push backis called the reaction force, which is felt as friction.But when running on a smooth floor the reactionforce isn’t there. Mostly we do not notice the reaction force except when it is not there.

All forces come in pairs. Pairs of forces are socommon that we often forget them. Some examples of times when pairs of forces are at workare walking, standing on a mattress, stepping off aboat, and the use of a jet engine. The direction offorces can be shown by arrows in diagrams.

In rockets and row boats, the forces are not balanced. One force is bigger and the object movesin the direction of the larger force. In these cases

122

Unbalanced forces4.10

reaction forceaction force

Stepping off a boat clearly shows the pairs of forces

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T AIM: To study action and reaction forcesA balloon rocket shows action and reaction forcesat work.Thread a drinking straw onto some stringor fishing line. Hold it or tie it so that it is tight.

Then blow up a balloon and tape it to the straw.Make sure you hold the end so the air does notescape. When you let go of the balloon, air comesout of the end.The action force of the escaping aircauses a reaction force that pushes the balloon inthe opposite direction.

As an extension: how can you make the balloonrocket go further or faster? Think of all the otherforces acting, and how you could increase them orreduce them.

straw nylon line

tape

escaping gasesballoon

the action and reaction forces are not balanced,and the rocket or row boat moves.

In a horse pulling a cart along a level road, thereare the weight and reaction forces to consider. Ifthe reaction forces are not strong enough, thenthe cart will sink into the ground and becomebogged. If the pulling force due to the horse is notgreater than the friction force, the cart will notmove forwards.

The concept of action and reaction is the thirdof Newton’s laws of motion. It is often worded as:‘For every action, there is an equal and opposite reaction.’ It can also be stated as: ‘If one object pushes onanother object, then the other object pushes back withequal force.’

Unbalanced forces cause motion. Firing a rocket motor, accelerating a car, and rowing a boatare all examples of unbalanced forces.

pulling forcedue tohorse

frictionforcepulls againstmotion

weight force

reaction force

The forces on a horse and cart

The arrows show the action and reaction forces

MOTION 123

In a car, the unbalanced force is friction. As a car rounds a corner, friction between the roadand tyres holds the car on the road. When thereaction force is exceeded, due to poor tyres or awet road, the car will skid.

The problem of action and reaction is important for astronauts. When they are weightless they float in their capsule. Their bodies do not exert a weight force, because thereis no gravity. To move across the cabin, the astronaut will push gently off one wall and floatover towards the opposite wall. Push too hardand the astronaut will accelerate to a high speed,

and will travel at this speed until crashing into theopposite wall.

If the astronaut goes for a ‘space walk’ there isnothing to push off. Astronauts have no way ofmoving. If they carried a rock they could move.By throwing the rock in one direction, theywould move in the opposite direction. The forceon the rock and the force on the astronaut wouldbe the same, but opposite.

Astronauts in space do not carry rocks. Tomove in space, astronauts have cylinders of compressed gas attached to their space suits. Tomove towards the spaceship they squirt the gasaway from the spaceship. This acts like a small jetengine. Again, action and reaction applies.

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Reaction ______ are all around us. When you _____ on the _____, you exert a ______ force _________. The _____pushes _______ against the weight _____. This invisible ______ force is called the ________ force.In _______ and ___ _____, the forces are not ________. One force is ______ and the ______ moves in the_________ of the ______ force.To move in _____, astronauts have _________ of __________ gas attached to their _____ suits. To move _______the _________ they squirt the gas ____ from the spaceship.

QUESTIONS

1 What, in your own words, is Newton’s third law ofmotion? Rewrite the meaning of the law in easy-to-understand language.

2 Floors become slippery if they are covered in water.Explain why, using the words ‘action’ and ‘reaction’.

3 Explain the idea of action and reaction in thesedrawings.

4 When people walk through sand they leave footprints.What would happen if the sand had a larger reaction force?

5 Why does your mattress bend in the middle when youstand on it, but a metal bench does not?

6 People rowing a boat always face backwards. Why is this?

7 An astronaut floating in deep space sees herspaceship in the distance. She is carrying an aerosolcan of oil and a heavy spanner. How can she get backto her spaceship?

The reaction force from throwing the rock makes the astronaut move

A rocket engine applies an unbalanced force to accelerate a rocket clear of the atmosphere

Isaac Newton devised the laws of motion in the1600s when the fastest known speed was on a galloping horse. He used his imagination andknowledge to explain how all objects moved. Hisideas can be applied to many things, including toycars, sports, space probes, rides in fun parks, transportation and the occupants in cars.

Most people travel by car and bus. This allowsus to experience the forces described in Newton’slaws of motion that Newton could only imagine.In making cars safer, designers and engineers mustremember Newton’s laws.

One of the major causes of death and injury inAustralia is traffic accidents. In order to reduce thenumber of deaths and injuries, road safety peopleare working towards:1 reducing the number of accidents (by having

safer roads and safer cars, by educating drivers of the dangers of driving too fast anddriving when fatigued, and by increasing theskills of drivers).

2 reducing the injuries to people involved in accidents. This is done by educating people towear seat belts, and by installing air bags andcrumple zones in cars. Again driver education isvery important.When a car stops suddenly the occupants keep

moving. They have inertia. (This is Newton’s firstlaw.) If the occupants are not restrained in theirseats they can move through the windscreen.Modern cars are fitted with seatbelts. Seatbeltshold the occupants in their seats and absorb theforces resulting from inertia. There are severepenalties for drivers and passengers who do notwear seatbelts.

If a car is hit from behind, a person’s head canfly backwards as the car is jolted forwards. Again,their head has inertia. Car seats have headrests toreduce the whiplash caused by inertia.

If a car travelling at high speed stops suddenly,it has a large deceleration. A large decelerationneeds a large force. (This is Newton’s second law.)Large forces applied to the occupants of cars causeinjuries, such as bruises and broken bones. Manysafety features in modern cars are designed toreduce these forces. The driver and passengersneed to decelerate more slowly in a crash to havefewer injuries.

If a car crashes into a solid barrier, the car crumples to absorb the energy. The bumpers andfront and back panels of the car crumple in adesigned manner. The car decelerates slower, andthe occupants have fewer—if any—injuries.

Air bags in cars have a similar purpose. Byinflating when the car suddenly slows down, theinertia and deceleration forces on the occupantsare absorbed as their bodies hit a soft bag of air.Car makers crash their cars into solid barriers totest them. Crumple zones and air bags are testedusing dummies. Instruments measure the forcesand deceleration on the dummies and in differentparts of the car.

Anti-lock brake systems (ABS) stop the wheelsfrom skidding in sudden braking. When a carskids, the friction with the road is lost. Newton’sthird law, which explains braking and cornering,relies on the action force of the tyres on the roadand the reaction force of the road on the tyres.Well-maintained tyres, with a deep tread, areneeded to gain the maximum friction. Wet roadshave less friction than dry roads.

The stopping distance of cars depends on thetyres and road surface. It also depends on thespeed of the car in an unexpected way. A car travelling twice as fast takes four times longer tostop. This is shown by the diagrams on page 125.Note that the reaction time is the time for the driver to react to the danger. It is the time between

124

Car safety4.11

A crash test can find out how effective a car’s crumple zones are

MOTION 125

seeing the danger and moving their foot from the accelerator to the brake. Reaction time is between0.5 and 1.0 second. During this time the car is stilltravelling at its original speed. The distance thatthe car travels during the reaction time is calledthe reaction distance. The braking distance is thedistance it takes the car to stop when the brakesare applied. The total distance is the stopping distance.

Alert drivers have fewer accidents than tiredand drug-affected drivers. This is because theirreaction times are faster, and they can foreseedangerous situations developing. The effects ofalcohol and fatigue are to lower concentrationand increase reaction times. Remember the slogan: Stop. Revive. Survive.

Encourage safe and courteous driving by yourparents and friends. Make sure they remain alertand do not drive when tired. Use their skills to

help you become a better driver when you getyour driver’s licence.

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When a car stops ________ the occupants keep ______. They have _______. _________ hold the occupants in their_____ and absorb the ______ resulting from _______.If a car travelling at ____ _____ stops ________, it has a large ____________. The ______ and __________ needto __________ more ______ in a crash to have fewer ________.Car makers _____ their cars into _____ barriers to ____ them. _______ zones and ___ bags are tested using_______.Anti-lock _____ systems stop the ______ from ________ in sudden braking. When a car _____ the ________ withthe ____ is lost.A car travelling _____ as fast takes ____ times longer to ____. The effects of _______ and _______ are to lower _____________ and increase ________ times. Remember the______: Stop. ______. _______.

QUESTIONS

1 List some safety features found on cars. Explain thefunction of each one.

2 Which of Newton’s laws of motion are being appliedin these situations?a Seatbelts hold passengers in their seats.b ABS stops skids in sudden braking.c Air bags decelerate passengers gently.d Tread on tyres increases friction with the road.e Headrests on seats reduce whiplash.f Crumple zones reduce the severity of sudden

impacts.

3 What are the meanings of reaction time, reactiondistance, braking distance, and stopping distance?

4 How is the stopping distance of a car related to itsspeed? How long does it take a car to stop if

travelling at 100 km/h? How would this change for atired driver, a wet road, poor tyres and a drunk driver?

5 Why do alert drivers have fewer accidents than tireddrivers?

6 Use the photograph of thecrash test dummy toanswer these questions.a How does the steering

wheel and airbagprevent injury to aperson in a head-oncollision?

b In what type ofcollision would the head restraint on the back ofthe seat help to prevent injury to a driver orpassenger? Why?

c Which commonly used safety feature is notpresent in this photograph?

204 m

157 m

101 m

77 m

38 m

14 m

6 m10m

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reaction distance

braking distance

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Stopping distances at different speeds

Crash test dummy

Review and Research

126

Newton’s LawsReview questions

1 Match the word with its meaning. Select fromthese words: acceleration, air bag, deceleration,force, friction, inertia, mass, reaction, seat belts,weightless.a tendency to stay still or keep movingb push or pull that changes motionc force that pushes back against another forced a change in speed, either faster, slower or

changing directione slowing down, a negative accelerationf the amount of matter or atoms in an objectg having no weight force because there is no

gravityh bag that inflates to protect car occupants in

collisionsi hold car occupants in seats to overcome

effects of inertiaj force that opposes motion by slowing moving

objects

2 According to Newton’s first law, an object willcontinue with constant speed unless acted uponby an unbalanced force.a What do you call the tendency of an object to

keep moving?b What force slows all moving objects?c What do you call a change in velocity?d What is an unbalanced force?

3 According to Newton’s second law, the size ofthe acceleration increases with the force anddecreases with the mass.a What is mass?b What is a negative acceleration called?c If the force is increased, and mass does not

change, how does this change theacceleration?

d If the mass is increased, and force isunchanged, how does this change theacceleration?

4 According to Newton’s third law of motion, for every action there is an equal and oppositereaction.a What is a reaction force?b When we stand on a hard floor, we exert a

weight force. Why don’t we crash through thefloor?

5 What is the meaning of the equation F = ma?What are the units of each quantity?

6 The diagram below shows a train traveller with aglass of orange juice.

a Explain what is happening with the motion ofthe train.

b What do we call the tendency of the orangejuice to move as the motion of the trainchanges?

c How will the level of the orange juice changeas the train rounds a right-hand bend?

7 The diagram shows a car, and a car towing acaravan.

a Which takes the greater effort to stop?b When travelling at 80 km/h on the highway,

which driver should keep a greater distanceaway from the car in front? Explain why.

c Which car would accelerate slower from rest?Which of Newton’s laws of motion explains this?

8 The direction of forces is often shown by anarrow.

a There are four forces acting on the boat. Theyare the force of the engine, the weight force,the reaction force (buoyancy) and friction.Redraw the diagram into your note book andshow the direction of these forces acting onthe boat.

b One pair of these forces isbalanced, and one pair isunbalanced. Identify them.

c The drawing shows a personpushing a wheelbarrow.Show the forces acting onthe wheelbarrow and the person.

MOTION 127

9 The drawing shows an accelerometer. It consistsof coloured liquid between two sheets of plastic.It is moving in the direction shown.a Is the accelerometer speeding up or slowing

down?

b What would be the level of the liquid if itwas travelling at a constant speed?

Research questions

1 Select one of the following scientists, who have been associated with helping people tounderstand motion, forces and acceleration.What contribution has the scientist made to our understanding of motion?Isaac Newton, Galileo Galilei, Albert Einstein

2 Modern cars have many safety devices, such asanti-lock brakes, air bags, energy absorbingsteering column and improved traction. On theoutside, there are crumple zones, energyabsorbing bumpers, shatterproof glass and evensealed (leak-proof) batteries.Select two of these safety features, and

describe how they help to reduce injury to theoccupants of cars in collisions.

3 What are g-forces? How do they make rides infun parks exciting, and how do they affect theperformance of pilots in military combataircraft?

4 What is weightlessness? How does it happen?What is it used for? How does it affect people?How do astronauts who have lived on theInternational Space Station, or Skylab, reactwhen they return to Earth? Consider their bonemass, muscle bulk, and difficulty in breathingnormal air.

5 How is speed measured? How do scientists knowthe speed of the Earth, or the speed of anasteroid hurtling through space? How is thespeed of a Lleyton Hewitt serve measured? How do they measure the speed of a CD in acomputer drive? Explain the science ofmeasuring speed.

Extension experiments

Aim: To find out how action and reaction apply tospring balances

a The diagram on the leftshows two identicalspring balances. Whatis the reading on eachbalance?

b If you placed two bathroom scales on top of each other,

what would be the reading on each when you stood on them? Explain.

Aim: To use a ticker timer to find the acceleration ofa toy carWhen the equation F = ma is rearranged to make mthe subject, it becomes m = Fa. You can find themass of an object by measuring the force andacceleration. The force is the weight force of amass hanging over the bench, and the accelerationcan be found by a ticker timer. The ticker timerneeds to be calibrated.

Using a toy car or physics trolley, find theforce and acceleration. Divide these to find themass. Check the mass with a balance. Hint: UseSI units throughout—metres, kilograms, seconds.

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Ticker timertape

to AC power supply

force dueto gravity

track tilted a little to allow for friction

Accelerometer

128 MOTION

MotionReview questions

1 Write ‘True’ or ‘False’ for each statement.a If a car travels 30 km in an hour, its

average speed has been 60 km/h.b Acceleration is measured in the unit

called newton.c Friction is the force that slows moving

objects.d Inertia makes moving objects go faster.e A force can be described as a push or pull or

twist.f When there is no air resistance, a heavy stone

falls faster than a light stone.g Balanced forces are where two forces push in

opposite directions and cancel each other.h An object is being accelerated if it is

changing direction.i The same force will produce a large

acceleration if the mass is small, and a smallacceleration if the mass is large.

j Speed can change direction, but velocity isalways in the one direction.

2 What is the difference between speed andvelocity? Which is the best to use to explain the motion of an aeroplane and the motion of a compact disc in a CD player?

3 A ticker timer leaves 20 dots per second. Drawnbelow are two actual size traces left by thesame ticker timer.a What is the speed recorded by tape 1?b Is the motion of the object recorded by tape

2 accelerating or decelerating?

4 Complete this information for a falling ball:

Time (s) 0 1 2 – – – 6

Speed (m/s) 0 10 20 30 – 50 –a What is the speed gained every second?b What is the acceleration? Include the correct

units.

5 Complete the missing information and find theacceleration due to gravity on this distantplanet.

Time (s) 0 1 2 3 4 5 6Speed (m/s) 0 2 4 – – – –Total distance (m) 0 1 4 9 – 36 –

a What is the acceleration?b How does the total distance travelled depend

on the time?

6 The distance–time graph drawn below shows themotion of a small vehicle on a railway track.

a What is the distance travelled in the first 100seconds?

b At what times was the vehicle stationary?c What was the greatest distance from the

starting point that the vehicle moved?d What is the total distance travelled?e When was the vehicle moving away? When

was it returning to the start?

7 The graph is a velocity–time graph of a race cardriving along a smooth straight road.

a What was the greatest speed reached?b How did the speed change between 10 s

and 15 s?c When was the car accelerating and

decelerating?d Explain the motion of the car. Include times

and speeds in your description.

8 Two bicycle riders, Jamie and Beppie, weretimed as they passed students standing at 5 mintervals. The results are shown in the table onpage 129.

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Time (s)0 50 100 150 200 250 300 350 400

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MOTION 129

Distance Time for Time fortravelled (m) Jamie (s) Beppie (s)5 m 2.35 2.7510 m 3.68 4.9815 m 4.40 6.7320 m 5.05 8.3825 m 5.80 10.0330 m 6.43 11.9035m 7.15 13.5240 m 7.84 15.2145 m 8.43 16.9150 m 9.07 18.61

Graph the results for Jamie and Beppie as adistance–time graph. The time axis is the x-axis. Draw a ‘line of best fit’ from 15 m to 50 m. A line of best fit is a straight line that averages out the inconsistencies in the measurements. In maths, the slope of a line is the rise dividedby the run, or y-axis over x-axis. In calculatingmotion, velocity = distance, which is the

timesame as the slope of this graph. Use the line of best fit for your calculations.

a What is Jamie’s final speed?b What is Beppie’s final speed?c How long did it take for Jamie to reach his

maximum speed?d How long did it take for Beppie to reach her

maximum speed?e What is the connection between steepness of

the graph and speed?Supposing we had a police radar, we couldmeasure the speed at each second. Graph thesepretend results for speed and time.Time (s) Jamie’s speed Beppie’s speed

(m/s) (m/s)1 2.5 0.72 5.0 1.43 6.25 2.14 6.25 2.85 6.25 2.86 6.25 2.8

f What is the connection between speed, timeand acceleration?

g How could you find acceleration from thisgraph?

h What is the acceleration of each rider in thefirst second of their journey?

Thinking questions

1 An astronaut is standing on a planet where theacceleration due to gravity is 1 m/s2. A largerock of mass 10 kg, 10 m directly above him,begins to fall.a The astronaut takes 5 s to move sideways out

of the path of the rock. Will he move beforethe rock hits him? (Hint: Draw a table like the one in Review Question 5).

b What will be the force when the rock hits himor the ground?

2 The drawings show some students standing onor riding skateboards. Give a reason for each ofthe observations.a When the girl

pushes theskateboard, therider moves. Whenthe girl is standingon a skateboard,they both move.

b When the boypushes theskateboard, itmoves away.When there aretwo people onthe skateboard, it moves away half as fast.

3 Two skateboards are side by side at the top of aslope. One has two students standing on it andthe other has one student on it. The studentsare all the same weight. a The skateboards start rolling down the slope

at the same time. Which one reaches thebottom of the slope first? Explain why.

b Both skateboards (and their riders) crash into a fence at the bottom of the slope.Which would exert a greater force on thefence? Explain why.

Word checktrundle wheel velocity instantaneousspeedometer stroboscopic circularpendulum acceleration parachutisttachograph gyroscope inertiagravitational friction decelerationreaction whiplash

Concept mapDraw a concept map of the main ideas in thischapter.

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