3. What is pressure?
Why would a lady in high heels standing on your foot hurt more than an elephant standing on your foot?
The elephant has a larger weight than the lady, but the contact area between its feet and the floor is far larger.

4. High pressure and low pressure
A force spread over a larger area produces a lower pressure.
Photo credit: © 2008 Jupiterimages Corporation
Which type of footwear would be better for standing on a muddy field – the flat-soled shoes or high-heeled boots?

5. High pressure and low pressure
The shoes have flat soles, so they spread the person’s weight over a large surface area.
These boots exert a low pressure on the ground.
In contrast, the soles of the high-heeled boots have a smaller surface area and so exert a higher pressure.
These shoes are likely to sink into soft ground.
Photo credit: © 2008 Jupiterimages Corporation

6. f p a force pressure = area Calculating pressure
Pressure is the force per unit area and is calculated using this formula: f p a
pressure =
area
force
This can also be represented in a formula triangle.
Pressure is measured in newtons per square metre (N/m2), which are also called pascals (Pa).
Pressure can also be measured in newtons per square centimetre (N/cm2).

7. Pressure calculation
How much pressure is the woman exerting on the floor?
Weight of woman = 500 N
Area of contact with floor = 20 cm2
Force
Pressure =
Area
Photo credit: © 2008 Jupiterimages Corporation
500 = 20
= 25 N/cm2

8. Using low pressure
A force spread over a large area produces low pressure. This is used in skis and snowboards.
The large surface area of the board means the snowboarder exerts very little pressure on the snow.
This means he slides over the top of the snow and does not sink into it.

9. Using high pressure
A force concentrated on a small area produces high pressure. This is used in things like pins and sharp knives.
The force applied to the tiny surface area of the end of a pin creates a high enough pressure to puncture even wood.
Photo credit: © 2008 Jupiterimages Corporation
The narrow blade of a knife means that it exerts a high pressure, making it easier to cut fruit and vegetables.

10. Investigating pressure10

11. Pressure – highest to lowest

13. low pressure high pressure
Pressure in a liquid
Pressure in a liquid acts in all directions and increases with depth.
You can investigate this by using a needle to make holes at different heights in a plastic bottle, before filling it with water. The water escaping from the lower holes will travel further because it is at a higher pressure.
low pressure
high pressure

14. Overcoming pressure underwater
When entering deep water, a diver will experience pressure from all sides.
At greater depths, this pressure becomes too much for the body to handle.
So how do we overcome this pressure?
Submarines use extremely strong materials to withstand the huge forces. This means some can dive to the very deepest points of the ocean.
Photo credit: © 2008 Jupiterimages Corporation

15. Pressure inside all parts of the hydraulic system is the same.
Hydraulic 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 the parts mechanically.
All hydraulic systems use two pistons linked via a pipe which carries special oil called hydraulic fluid.
force
applied
here
force transferred
here
Pressure inside all parts of the
hydraulic system is the same.

16. Hydraulic brakes
All hydraulic brake systems (like those used in cars) use a small master piston and bigger slave pistons.
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 of the car. 16

17. Hydraulic car brake – labelling the parts

18. Hydraulic brake – pressure equations
The pressure exerted by the master piston on the hydraulic fluid can be calculated using this equation:
force applied
pressure =
area of master piston
The pressure is transferred to the slave pistons, so the
force exerted by the slave piston can be calculated using:
force exerted
pressure =
area of slave piston
force exerted = pressure × area of slave piston
The slave piston has a larger area than the master piston,
so the force exerted by the slave pistons is greater than the force exerted by the driver on the brake pedal.

19. What causes pressure in gases?
Worksheet 1 accompanies this slide.
The worksheet involves a role play to model pressure in a container of gas, along with questions to check understanding.

20. Pressure: summary

22. A force acting on an object can cause it to turn about a pivot.
Force and rotation
A force acting on an object can cause it to turn about a pivot.
100 N
pivot
What happens to the see-saw when a force is applied on the left-hand side?
Does the see-saw turn? If so, clockwise or anticlockwise?

23. Force and rotation – a moment
The left-hand side of the see-saw moves downwards when a force is applied to it – this is an anticlockwise turn.
100 N
The turning effect of a force is called a moment.

24. Using moments
A spanner is a lever that can be used to unscrew a nut.
The spanner exerts a moment or turning force on the nut.
pivot
distance from force to pivot
force
force
If the moment is big enough, it will unscrew the nut. If not, there are two ways of increasing the moment.

25. Using moments – 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
distance from force to pivot
force
If the same force is applied over a greater distance, a larger moment is produced.

26. Using moments – increasing the moment
2. Increase the force applied – push/pull harder or get someone stronger to do it!
pivot
distance from force to pivot
force
If a greater force is applied over the same distance, a larger moment is produced.

27. Worksheet 2 accompanies this section.
The worksheet involves an experiment using a ruler and ring clamp stand that allows students to investigate moments for themselves.

28. f d Moment equation The moment of a force is given by the formula:
moment = force (N) × distance from pivot (cm or m)
This can also be represented in a formula triangle:
moment f d
Moments are measured in newton centimetres (Ncm) or newton metres (Nm).

29. Moment calculation
The counterweight on the trebuchet weighs 300 N and is attached to the short arm. It is 1 m from the pivot. It exerts a clockwise moment. What is the size of this moment?
moment = 300 × 1 = 300 Nm
3 m
1 m
100 N
300 N
pivot

30. Principle of moments
The lead shot on the trebuchet weighs 100 N and is attached to the long arm. It is 3 m from the pivot. It exerts a clockwise moment. What is the size of this moment?
moment = 100 x 3 = 300 Nm
3 m
1 m
100 N
300 N
pivot

31. Principle of moments
If the anticlockwise moment and clockwise moment are equal, then the trebuchet is balanced. This is known as the principle of moments.
When something is balanced about a pivot:
total clockwise moment = total anticlockwise moment
3 m
1 m
100 N
300 N
pivot

32. Principle of moments
What happens if the counterweight is increased to 1000 N?
The moments will no longer be balanced, so the trebuchet will be able to fire.
3 m 1m 1m
100 N
1000 N

33. Both moments are equal, so the see-saw is balanced.
Principle of moments
The principle of moments can be investigated using 10 g masses with this balance. 10 g exerts a force of 0.1 N.
Anticlockwise
moment
Clockwise moment
= 0.1 × 7
= 0.7 Ncm
= (0.1 × 3) + (0.1 × 4) = 0.7 Ncm
Both moments are equal, so the see-saw is balanced.

34. Principle of moments – calculation
Two girls are sitting on opposite sides of a see-saw. One girl weighs 200 N and is 1.5 m from the pivot. How far from the pivot must her 250 N friend sit if the see-saw is to balance?
200 N
250 N
1.5 m
? m
total clockwise moment = total anticlockwise moment
200 N × 1.5 m = 250 N × distance
200 × 1.5 = distance
250
= 1.2 m

35. How do tower cranes work?
Tower cranes are essential at any major construction site.
trolley
load arm
counterweight
loading platform
tower
Concrete counterweights are fitted to the crane’s short arm. Why are these needed for lifting heavy loads?

36. Moment calculation – crane
If the crane below is balanced, how heavy is the load?
3 m
6 m
10,000 N ?
moment of = moment of
load counterweight

37. Moment calculation – crane
Moment of load
= load × distance from tower
= ? × 6
Counterweight moment
= counterweight × distance
= 10,000 × 3
= 30,000 Nm
6 m
3 m
Moment of load
= moment of counterweight
? × 6
= 30,000
10,000 N ?
5,000 N
= 30,000 ? 6 ? =
5,000 N

38. Crane operator activity

40. Glossary
Glossary:
counterbalance – A weight used to balance another weight.
effort – The force applied to a lever.
hydraulics – The use of liquid to transmit pressure from one place to another.
lever – A simple machine that moves about a pivot and makes work easier by increasing the size of a force.
load – The force moved when using a lever.
moment – The turning effect of a force. It equals the force multiplied by the distance from the pivot.
pascal – A unit of pressure (Pa). 1 Pa = 1 newton per square metre (N/m2).
pivot – The point around which a lever turns.
pressure – The force pushing on a given area.

41. Anagrams

42. Pressure – true or false?

43. Match the definition

44. Multiple-choice quiz