chapter: motion, acceleration, and forces comp .pdf¥a reference point is needed to ... distance...

Chapter: Motion, Acceleration,

and Forces

Table of Contents

Section 3: Motion and Forces

Section 1: Describing Motion

Section 2: Acceleration

• Are distance andtime important indescribing runningevents at the track-and-field meets inthe Olympics?

Motion1

Describing Motion

• Distance and time are important. In order towin a race, you must cover the distance in theshortest amount of time.

• How would youdescribe the motion ofthe runners in the race?

Motion

Describing Motion

1

• You don’t always need to see somethingmove to know that motion has taken place.

• A reference point is needed to determinethe position of an object.

Motion and Position

Describing Motion

• Motion occurs when an object changes itsposition relative to a reference point.

• The motion of an object depends on thereference point that is chosen.

1

• If you are sitting in a chair reading thissentence, you are moving.

• You are not moving relative to your deskor your school building, but you aremoving relative to the other planets in thesolar system and the Sun.

Relative Motion

Describing Motion

1

• An important part of describing the motionof an object is to describe how far it hasmoved, which is distance.

• The SI unit of length or distance is themeter (m). Longer distances are measuredin kilometers (km).

Distance

Describing Motion

1

Distance

Describing Motion

• Shorter distances are measured in centimeters(cm).

1

• The runner travels 50 m inthe original direction(north) plus 30 m in theopposite direction (south),so the total distance sheran is 80 m.

Displacement

Describing Motion

• Suppose a runner jogs to the 50-m mark andthen turns around and runs

back to the 20-m mark.

1

• Displacement is thedistance and direction ofan object’s change inposition from the startingpoint.

Displacement

Describing Motion

• Sometimes you may want to know not onlyyour distance but also yourdirection from a referencepoint, such as from thestarting point.

1

• The length of the runner’sdisplacement and thedistance traveled would bethe same if the runner’smotion was in a singledirection.

Displacement

Describing Motion

1

• You could describe movement by thedistance traveled and by the displacementfrom the starting point.

• You also might want to describe how fastit is moving.

Speed

Describing Motion

• Speed is the distance an object travels perunit of time.

1

• Any change over time is called a rate.

• If you think of distance as the change inposition, then speed is the rate at whichdistance is traveled or the rate of change inposition.

Calculating Speed

Describing Motion

1

Calculating Speed

Describing Motion

• The SI unit for distance is the meter and theSI unit of time is the second (s), so in SI,units of speed

aremeasured inmeters persecond(m/s).

1

• Sometimes it is more convenient to expressspeed in other units, such as kilometers perhour (km/h).

Calculating Speed

Describing Motion

1

• Suppose you are in a car traveling on a nearlyempty freeway. You look at the speedometerand see that the car’s speed hardly changes.

• If you are traveling at a constant speed, youcan measure your speed over any distanceinterval.

Motion with Constant Speed

Describing Motion

1

• Usually speed is not constant.

Changing Speed

Describing Motion

• Think aboutriding abicycle for adistance of 5km, as shown.

1

Changing Speed

Describing Motion

• How would you express your speed on such atrip? Would

you use yourfastest speed,your slowestspeed, or somespeed betweenthe two?

1

• Average speed describes speed of motionwhen speed is changing.

Average Speed

Describing Motion

• Average speed is the total distance traveleddivided by the total time of travel.

• If the total distance traveled was 5 km andthe total time was 1/4 h, or 0.25 h. Theaverage speed was:

1

• A speedometer shows how fast a car is goingat one point in time or at one instant.

Instantaneous Speed

Describing Motion

• The speed shown on aspeedometer is theinstantaneous speed.Instantaneous speedis the speed at a givenpoint in time.

1

• When something is speeding up or slowingdown, its instantaneous speed is changing.

Changing Instantaneous Speed

Describing Motion

• If an object is moving with constant speed,the instantaneous speed doesn’t change.

1

• The motion of anobject over aperiod of time canbe shown on adistance-timegraph.

Graphing Motion

Describing Motion

• Time is plotted along the horizontal axis ofthe graph and the distance traveled isplotted along the vertical axis of the graph.

Click image to play movie.

1

• On a distance-time graph, the distance isplotted on the vertical axis and the time onthe horizontal axis.

Plotting a Distance-Time Graph

Describing Motion

• Each axis must have a scale that covers therange of number to be plotted.

1

• Once the scales for each axis are in place,the data points can be plotted.

Plotting a Distance-Time Graph

Describing Motion

• After plotting the data points, draw a lineconnecting the points.

1

• Speed describes only how fast something ismoving.

Velocity

Describing Motion

• To determine direction you need to knowthe velocity.

• Velocity includes the speed of an objectand the direction of its motion.

1

• Because velocity depends on direction aswell as speed, the velocity of an object canchange even if the speed of the objectremains constant.

Velocity

Describing Motion

• The speed of this carmight be constant,but its velocity is notconstant because thedirection of motionis always changing.

1

Acceleration, Speed and Velocity

• Acceleration is the rate of change ofvelocity. When the velocity of an objectchanges, the object is accelerating.

• A change in velocity can be either a changein how fast something is moving, or a changein the direction it is moving.

• Acceleration occurs when an object changesits speed, its direction, or both.

2Acceleration

Speeding Up and Slowing Down

• When you think of acceleration, youprobably think of something speeding up.However, an object that is slowing down alsois accelerating.

• Acceleration also has direction, just asvelocity does.

Acceleration

2


Acceleration

• If the acceleration is in the same direction asthe velocity,

the speedincreases andtheacceleration ispositive.

2


Acceleration

• If the speed decreases, the acceleration is inthe opposite

direction fromthe velocity,and theacceleration isnegative.

2

Changing Direction

• A change in velocity can be either a changein how fast something is moving or a changein the direction of movement.

• Any time a moving object changes direction,its velocity changes and it is accelerating.

Acceleration

2

Changing Direction

• The speed of thehorses in thiscarousel isconstant, but thehorses areacceleratingbecause theirdirection ischangingconstantly.

Acceleration

2

Calculating Acceleration

• To calculate the acceleration of an object, thechange in velocity is divided by the length oftime interval over which the change occurred.

Acceleration

• To calculate the change in velocity, subtractthe initial velocity—the velocity at thebeginning of the time interval—from the finalvelocity—the velocity at the end of the timeinterval.

2


• Then the change in velocity is:

Acceleration

2


• Using this expression for the change invelocity, the acceleration can be calculatedfrom the following equation:

Acceleration

2


• If the direction of motion doesn’t changeand the object moves in a straight line, thechange in velocity is the same as the changein speed.

Acceleration

• The change in velocity then is the final speedminus the initial speed.

2

Calculating Positive Acceleration

• How is the acceleration for an object that isspeeding up different from that of an objectthat is slowing down?

Acceleration

• Suppose a jet airliner starts at rest at the endof a runway and reaches a speed of 80 m/s in20 s.

2


• The airliner is traveling in a straight linedown the runway, so its speed and velocityare the same.

Acceleration

• Because itstarted fromrest, itsinitial speedwas zero.

2


• Its acceleration can be calculated as follows:

Acceleration

2


Acceleration

• The airliner isspeeding up, so thefinal speed isgreater than theinitial speed andthe acceleration ispositive.

2

Calculating Negative Acceleration

Acceleration

• The final speedis zero and theinitial speedwas 3 m/s.

• Now imagine that a skateboarder is movingin a straight line at a constant speed of 3 m/sand comes to a

stop in 2 s.

2


• The skateboarder’s acceleration is calculatedas follows:

Acceleration

2


Acceleration

• The accelerationalways will bepositive if an objectis speeding up andnegative if the objectis slowing down.

• The skateboarder is slowing down, so thefinal speed is less than the initial speed andthe acceleration is

negative.

2

Amusement Park Acceleration

• Engineers use the laws of physics to designamusement park rides that are thrilling, butharmless.

Acceleration

• The highestspeeds andaccelerationsusually areproduced onsteel rollercoasters.

2


• Steel roller coasters can offer multiple steepdrops and inversion loops, which give therider large accelerations.

Acceleration

• As the rider moves down a steep hill or aninversion loop, he or she will acceleratetoward the ground due to gravity.

2


• When riders go around a sharp turn, theyalso are accelerated.

Acceleration

• This acceleration makes them feel as if aforce is pushing them toward the side ofthe car.

2

What is force?

• A force is a push or pull.

• Sometimes it is obvious that a force has beenapplied.

3Motion and Forces

• But other forces aren’t as noticeable.

Changing Motion

• A force can cause the motion of an object tochange.

Motion and Forces

• If you haveplayed billiards,you know thatyou can force aball at rest to rollinto a pocket bystriking it withanother ball.

3

Changing Motion

Motion and Forces

• The force of the moving ball causes the ballat rest to move in the direction of the force.

3

Balanced Forces

• Force does not always change velocity.

• When two or more forces act on an object atthe same time, the forces combine to form thenet force.

Motion and Forces

3

Balanced Forces

• The net force on the box is zero because thetwo forces cancel each other.

• Forces on an objectthat are equal in sizeand opposite indirection are calledbalanced forces.

Motion and Forces

3

Unbalanced Forces

• When two students are pushing with unequalforces in opposite directions, a net forceoccurs in the direction of the larger force.

Motion and Forces

3

Unbalanced Forces

Motion and Forces

• They are consideredto be unbalancedforces.

• The net force that moves the box will be thedifference between

the two forcesbecause they are inopposite directions.

3

Unbalanced Forces

• The students are pushing on the box in thesame direction.

Motion and Forces

• These forces arecombined, or addedtogether, becausethey are exerted onthe box in the samedirection.

3

Unbalanced Forces

Motion and Forces

• The net force thatacts on this box isfound by adding thetwo forces together.

3

Inertia and Mass

• Inertia (ih NUR shuh) is the tendency of anobject to resist any change in its motion.

Motion and Forces

• If an object is moving, it will have uniformmotion.

• It will keep moving at the same speed and inthe same direction unless an unbalanced forceacts on it.

3

Inertia and Mass

• The velocity of the object remains constantunless a force changes it.

Motion and Forces

• If an object is at rest, it tends to remain atrest. Its velocity is zero unless a force makesit move.

• The inertia of an object is related to its mass.The greater the mass of an object is, thegreater its inertia.

3

Newton’s Laws of Motion

• The British scientist Sir Isaac Newton(1642–1727) was able to state rules thatdescribe the effects of forces on the motionof objects.

Motion and Forces

• These rules are known as Newton’s laws ofmotion.

3

Newton’s First Law of Motion

• Newton’s first law of motion states that anobject moving at a constant velocity keepsmoving at that velocity unless an unbalancednet force acts on it.

Motion and Forces

• If an object is at rest, it stays at rest unlessan unbalanced net force acts on it.

• This law is sometimes called the law ofinertia.

3

What happens in a crash?

• The law of inertia can explain what happensin a car crash.

Motion and Forces

• When a car travelingabout 50 km/hcollides head-on withsomething solid, thecar crumples, slowsdown, and stopswithin approximately0.1 s.

3

What happens in a crash?

• Any passenger not wearing a safety beltcontinues to move forward at the same speedthe car was traveling.

Motion and Forces

• Within about 0.02 s (1/50 of a second) afterthe car stops, unbelted passengers slam intothe dashboard, steering wheel, windshield, orthe backs of the front seats.

3

Safety Belts

• The force needed to slow a person from 50km/h to zero in 0.1 s is equal to 14 times theforce that gravity exerts on the person.

Motion and Forces

• The belt loosens a little as it restrains theperson, increasing the time it takes to slowthe person down.

3

Safety Belts

• This reduces the force exerted on the person.

Motion and Forces

• The safety belt also prevents the person frombeing thrown out of the car.

3

Safety Belts

• Air bags also reduce injuries in car crashes byproviding a cushion that reduces the force onthe car’s occupants.

Motion and Forces

• When impact occurs, a chemical reactionoccurs in the air bag that produces nitrogengas.

• The air bag expands rapidly and then deflatesjust as quickly as the nitrogen gas escapes outof tiny holes in the bag.

3

chapter: motion, acceleration, and forces comp .pdf¥a reference point is needed to ... distance...

Documents