© boardworks ltd 2001 ks3 forces and motion. © boardworks ltd 2001 contents motion weight, mass...
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© Boardworks Ltd 2001
KS3
Forces and Motion
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Contents
Motion
Weight, mass and gravity
Balanced and unbalanced forces
Friction
Moments
Pressure and Hydraulics
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Distance, Time and Speed
To work out the speed of an object you need to know:
• the distance traveled
• how long it took to travel that distance
distance traveled
timetaken
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Use this equation:
SPEED = distance
time
Speed is measured in many different units, e.g. m/s, km/h, km/s, miles per hour.The units of distance and time used will give the units to be used for speed.
d
s x t
Formula triangle
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A boy travels from his home to the cinema, a distance of 10 km in 1 hour. Calculate his speed in km/h.
Examples
Speed in km/h =
d
s x t
Cover the quantity you want to calculate - s (speed)
d (distance in km)
t (time in h)
10 km
1 h=
= 10 km/h
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A boy travels from his home to the cinema, a distance of 10 km in 1 hour. Calculate his speed in m/s. (1km = 1000m)
Speed in m/s =
d
s x t
Cover the quantity you want to calculate - s (speed)
d (distance in m)
t (time in s)
10,000 m
3600 s=
= 2.8 m/s
Click for solution
You sometimes have to change the units in this type of problem - here is the same problem again
1x60x60
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Distance (km) = Speed (km/h) x time (h)
= 3.6 km/h x 2 h
= 7.2 km
Questions
1. A group set off from home and walk at an average speed of 3.6 km/h. How far would they travel in 2 hours? Give your answer in km.
d
s x t
Click for solution
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2. How long would it take a woman to walk 10 km if her average speed is 5.4 km/h ?
d
s x t
Time =
Time =
Time = 1.85 hours
distance
speed
10 km
5.4 km/h
Click for solution
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Experiments - Speed
1. Time how long it takes you to run 100m.
2. Then calculate your speed for the run.
SPEED (m/s) =
3. Repeat the experiment for each member of your group. What was the fastest speed for your group ?
Distance (m)
Time (s)
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Name distance (m) time (s) speed (m/s)
100
100
100
100
100
Results
Conclusion
The fastest member of the group with a speed of ______ was _______.
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Weight, mass and gravity
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Weight and MassWeight and mass are not the same.
Mass is the amount of matter in an object.
This will have the same value anywhere in the Universe including space.
Weight is a force and it is caused by the pull of gravity.
In fact, weight is the pull of gravity acting on a mass.
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Weight is a force so is measured in Newtons. Like other forces it has both magnitude and direction.
Mass is not a force, it is measured in kilograms.
A 1 kg mass will weigh less on the moon than it does on Earth.
This is because the force of gravity is less on the moon because the moon is smaller than the Earth.
An astronaut could jump 20 feet into the air on the moon because gravity is less.
However, he still has the same body, and the same mass, it just weighs less, because he is on the moon and gravity is weaker.
Weight and Mass
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Weight and MassSo, a scientist should never say “He weighs 50kgs”
but, should say
“He has a mass of 50kgs”,
or the scientist could say:
“the gravitational force acting on his mass is about 500 Newtons”.
This is the same as saying:
“his weight is about 500 Newtons”.
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Gravity
Gravity is an attractive force that acts between all masses. The force depends on the mass of the object.
All objects produce a gravitational force but it is only significant when the mass is about the size of a moon or planet.
Think about it:
When you jump the gravitation force of the Earth pulls you down. Your gravitation force pulls the Earth up!
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Force of gravity
Earth - large mass
Space shuttle- smaller mass
The force of gravity depends on the mass of the planet and how close you are.
Objects will have higher weight on Jupiter because it has a larger mass than Earth.
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Balanced and unbalanced
forces
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Forces
If you link two newton meters and pull equally hard from both ends, the forces recorded on both will be the same.
We say that the forces acting on the central hooks cancel each other out - they are equal in magnitude and opposite
in direction.
Because the forces are balanced the hooks do not move.
Balanced Forces
10 N10 N
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Forces
What happens if the pull on one end is harder than on the other?
The forces acting on the hooks are no longer balanced. Both hooks will start to move to the left, that is, their speed will
change. This is called acceleration.
Unbalanced forces lead to a change in speed or direction.
unbalanced Forces
11 N 10 Nmovement
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More Balanced and Unbalanced Forces
500 N500 N
Think of a car traveling at a constant 50 mph. The engine provides sufficient force to just overcome all the frictional forces that are acting to decrease the speed.
50 mph
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50 mph
Cross wind
Now a cross-wind acting on the car produces a sideways force.
This causes the direction of the car to change. This happens because the sideways forces on the car are
not balanced.
If the car turns left so that the wind is now BEHIND the car, what will happen to the speed?
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The air resistance will decrease because the car has a “tail wind” (it is being blown from behind).
This means the forces acting on the car are no longer balanced. The car will increase in speed (accelerate)
until the forces are balanced again.
500 N400 N
> 50 mph
500 N500 N
60 mph
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Summary
If the forces on an object are balanced :
• If it is stopped it will remain stopped.
• If it is moving then it will continue to move at the same speed.
In other words, it will continue to do what it is already doing without any change.
If the forces are unbalanced two things can happen
• The speed will change.
• The direction of motion will change.
This is called acceleration.
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Resultant Forces
The sum effect of more than one force is called the resultant force.
You can find out the resultant force by calculating the difference between opposing forces.
500 N400 N
100 N
A resultant force of 100 N is accelerating the car.
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5N5N
20N
1.
Find the resultant force:
10N
Click for solution
Resultant force = 20N -10N = 10N downThe block will accelerate down.
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2.5N
5N
5N
5N
Click for solution
Resultant force = 5N - 0N = 5N right.
The vertical forces are equal in size and opposite in direction so there is no resultant force in the vertical direction. The block will accelerate to the right.
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13N
3.
10N
20N
10N
3N7N
17N
Click for solution
Resultant force = 30 - 13 = 17N right.
The vertical forces are equal in size and opposite in direction so there is no resultant force in the vertical direction. The block will accelerate to the right.
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Friction
Friction always tries to slow moving object down. We say it opposes motion.
Friction is created whenever two touching objects or surfaces move past each other.
Friction also occurs when things move through air. This is called air resistance or drag.
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One more? Probably the most important…
FrictionOn the diagram label all sources of friction.
tyre and road
Brake pad and rim
Wheel bearingWheel bearing
Pedal bearing
Links in chain
Click for answers
Air resistance, or “Drag”
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400 N
300 N
Air Resistance or dragAir resistance is a type of friction caused when objects move through the air. Cars are designed so that they are streamlined. The flow of air around the body is made as smooth as possible so that air resistance is minimized.
Air resistance depends on:• the size of the car;• the shape of the car;• the speed of the car.
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Other Sources of Friction in Cars
One of the most important sources of friction in cars in that between the tyre and the road.
When the car brakes, the
The friction between the tyre and the road is affected by:
• inflation pressure of the tyre
• the road surface
• whether the surface is wet.
maximum possible amount of friction is desirable so that the car does not skid.
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Force and Rotation
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Force and Rotation
5N
A force acting on an object can cause it to turn about a pivot.
What would happen to the see-saw above ?
Would it turn? If so, clockwise or anti-clockwise?
pivot
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Force and Rotation
The left goes down - an anticlockwise turn.
pivot
A turning force is called a moment.
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Moments
Suppose you were trying to unscrew a nut using a spanner. The spanner exerts a moment or turning force on the nut.
If the moment is big enough it will unscrew the nut. If not there are 2 ways of increasing the moment.
Distance from force to pivot
Distance from force to pivot
PivotPivot
ForceForce
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Increasing the moment
1. Increase the distance from the force to the pivot - apply the force at the end or use a longer spanner.
Distance from force to pivot
Pivot
Force
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2. Increase the force applied - push/pull harder or get someone stronger to do it!
Distance from force to pivot
pivot
Force
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Moment
Moment = Force (N) x Distance (cm or m).
The moment of a force is given by the relationship:
Moments are measured in Newton centimeter (Ncm) or Newton metre (Nm).
moment
F x d
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pivot
500N
0.5m
A 500 N woman stands on one end of a see-saw. She is 0.5m from the pivot. What moment does she exert?
Moment = 500 x 0.5
= 250 Nm.
Click for solution
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Principle of Moments
The green girl exerts an anti-clockwise moment equal to ...
her weight x distance from pivot.
The yellow girl exerts a clockwise moment equal to...
her weight x distance from pivot.
pivot
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If the two moments are equal then the seesaw is balanced. This is known as the principle of moments.
When balanced
Total clockwise moment = total anti-clockwise moment
“c.m.” = “a-c.m.”
Principle of Moments
pivot
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The principle of moments can be investigated using the balance shown below with 10g masses:
Moment (left) = 7 x 10 = 70gcm
Moment (right) = (3 x 10) + (4x10) = 70gcm
Both moments are equal therefore the seesaw is balanced.
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Using Moments in Calculations
1. Two girls are on a seesaw. One weighs 200N and is 1.5m from the pivot. Where must her 150N friend sit if the seesaw is to balance ?. Click for solution
At balance
total “c.m.” = total “a-c.m.”
200 x 1.5 = 150 x distance
200 x 1.5
distance = 2 m
150= distance
“c.m.” = clockwise moment
“a-c.m.” =
anti-clockwise moment
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Pressure and Hydraulics
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Pressure
Pressure is exerted whenever a force is applied over an area.
Which one exerts the biggest pressure, 1 or 2?
1. 2.
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Case 1.
The arm applies a force onto a board via a finger tip.
The force applied produces a high pressure because the force acts over a small area.
1.
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2.
Case 2.
The arm applies the same force onto the board.
The force is now acting over a larger area - the area of the palm is greater than the finger tip.
Thus, a lower pressure is produced.
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Pressure
Pressure is measured in:Newtons per metre squared (N/m2) which is called
a PASCAL (Pa)
Pressure can also be measured in:Newtons per millimetre squared (N/mm2);Newtons per centimeter squared (N/cm2).
Pressure =Area
ForceP x A
F
Pressure is the force per unit area so is calculated using the expression shown below:
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The same force spread over a big area means low pressure.
Which shoes would you choose for walking over a muddy field?
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The boots on the right spread the weight over a larger area. Therefore, the pressure exerted on the ground
is low.
In contrast, fashion shoes have a smaller area and exert a higher pressure. These shoes are likely to
sink into soft ground.
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A force spread over a large area means low pressure, e.g. skis and snowboards.
Application of Pressure
The large surface area of the board means the boy exerts very little pressure
on the snow. He therefore slides over the top and does not sink in.
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A force concentrated on a small area means high pressure, e.g. razor blades, needles, high heeled shoes, spurs, ice skates, sharp knives.
Application of Pressure
On the cutting edge of a knife a very high pressure is exerted - this makes
it easier to cut.
The high pressure on the cutting edge of an ice-skate melts the ice and helps the skater slide across
the surface.
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Pressure in Liquids
In a liquid:
Pressure acts in all directions and
pressure increases with depth.
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High pressure
low pressure
The relationship between pressure and depth is shown by a water bottle with holes along its length.
Pressure (N/m2) = 10 N/Kg x depth (m) x density (Kg/m3)
The pull of gravity
The deeper you go, the higher the pressure
The denser the liquid, the heavier it is!
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HydraulicsHydraulic systems use the principle that pressure is transmitted throughout a liquid. They are used to transfer movement from one part of a machine to another without linking them mechanically.
All hydraulic systems use two pistons linked via a pipe carrying a special oil called hydraulic fluid.
Force applied
here Force transferred
here
Pressure inside all parts of the hydraulic system is the same
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Hydraulics Brakes
All hydraulic brake systems (eg in a car) use a small master piston and a bigger slave piston.
The master piston is used to apply a force. This puts the liquid under pressure.
The pressure is transmitted to the pistons on all four wheels.
Pressure = Force applied
Area master piston
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The slave piston always has a much larger area than the master piston. The force exerted can be calculated from the same equation:
So, a greater force is exerted by the brakes than the driver exerted on the pedal.
Pressure = Force exerted
Area slave piston
Force exerted = Pressure x Area slave piston
Much larger than master piston
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The Hydraulic Brake
Foot pedal
Master piston
Slave pistons
drum
Friction shoes
Hydraulic fluid
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The Hydraulic Brake - example
The car master piston has an area of 5cm2. If a force of 10N is applied to it, calculate the pressure created in the brake pipes. If the slave piston has an area of 50 cm2, calculate the force exerted on the brake disc.
At the master piston, P=F/A= 10/5 = 2 N/cm2
At the slave piston, F= PxA =2x50 = 100 N
(10 times the original force applied to the master piston).
Click for solution