© Boardworks Ltd 2001 KS3 Forces and Motion. © Boardworks Ltd 2001 Contents Motion Weight, mass and gravity Balanced and unbalanced forces Friction Moments

© Boardworks Ltd 2001

KS3

Forces and Motion


Contents

Motion

Weight, mass and gravity

Balanced and unbalanced forces

Friction

Moments

Pressure and Hydraulics


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


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


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


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


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


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


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)


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 _______.


Weight, mass and gravity


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.


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


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”.


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!


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.


Balanced and unbalanced

forces


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


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


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


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?


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


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.


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.


5N5N

20N

1.

Find the resultant force:

10N

Click for solution

Resultant force = 20N -10N = 10N downThe block will accelerate down.


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.


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.


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.


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”


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.


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.


Force and Rotation


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


Force and Rotation

The left goes down - an anticlockwise turn.

pivot

A turning force is called a moment.


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


PivotPivot

ForceForce


Increasing the moment

1. Increase the distance from the force to the pivot - apply the force at the end or use a longer spanner.


Pivot

Force


2. Increase the force applied - push/pull harder or get someone stronger to do it!


pivot

Force


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


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


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


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


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.


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


Pressure and Hydraulics


Pressure

Pressure is exerted whenever a force is applied over an area.

Which one exerts the biggest pressure, 1 or 2?

1. 2.


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.


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.


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:


The same force spread over a big area means low pressure.

Which shoes would you choose for walking over a muddy field?


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.


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.


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.


Pressure in Liquids

In a liquid:

Pressure acts in all directions and

pressure increases with depth.


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!


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


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


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


The Hydraulic Brake

Foot pedal

Master piston

Slave pistons

drum

Friction shoes

Hydraulic fluid


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

Documents

© Boardworks Ltd 2001 KS3 Forces and Motion. © Boardworks Ltd 2001 Contents Motion Weight, mass and gravity Balanced and unbalanced forces Friction Moments