1. KS3 Physics
9L Pressure and Moments

2. 9L Pressure and Moments Pressure Pressure in liquids Moments
Contents
9L Pressure and Moments
Pressure
Pressure in liquids
Moments
Summary activities

3. What is pressure?
Pressure is exerted whenever a force is applied over an area. 1. 2.
If the same force is applied in each picture, which arm exerts the highest pressure on the board?

4. High and low pressure 1. 2. The arm applies a force
to the board via a fingertip.
The force acts over a
small area and so
produces a high pressure.
The same force is now acting over a larger area – the palm has a greater surface area
than the fingertip.
A lower pressure is produced.

5. f p x a force pressure = area Calculating pressure
Pressure is the force per unit area
and is calculated using this formula:
p x a f
pressure =
area
force
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 millimetre (N/mm2); newtons per square centimetre (N/cm2).

6. Which type of pressure?
The same force spread over a larger area means a lower pressure.
Which type of shoes would be best for walking over a muddy field – flat soles or heels?

7. Which type of pressure?
The boots have flat soles and spread the person’s weight over a large surface area.
These boots exert a low pressure on the ground.
In contrast, the heeled shoes have a smaller surface area and so exert a higher pressure.
These shoes are likely
to sink into soft ground.

8. Using low pressure
A force spread over a large area means low pressure, e.g. skis and snowboards.
The large surface area of the board means the skier 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 means high pressure, e.g. high-heeled shoes, needles, ice skates, sharp knives.
The high pressure of the blade of an ice-skate melts the ice and helps the skater slide across the surface.
The narrow blade of a knife means that it exerts a high pressure and makes it easier to cut fruit and vegetables.

10. Under pressure low pressure high pressure
Why would a lady in high heels standing on your foot hurt more than an elephant standing on your foot?
This slide should have two cartoon drawings on it.
On the left should be a cartoon showing a zoo keeper and an elephant. The elephant is stepping on the zoo keepers foot and the zoo keeper looks in pain but not too hurt.
On the right is a cartoon showing two ballroom dancers and the women is stepping on the man’s foot with her stiletto heel. The man looks to be in agony with a bright red face etc…..
low pressure
high pressure

11. Pressure – highest to lowest?

12. 9L Pressure and Moments Pressure Pressure in liquids Moments
Contents
9L Pressure and Moments
Pressure
Pressure in liquids
Moments
Summary activities

13. Pressure in a liquid Pressure in a liquid: acts in all directions;
increases with depth.
A liquid can be used to transmit pressure from one place to another.

14. low pressure high pressure
Pressure in a liquid
The relationship between pressure and depth is shown by a water bottle with holes along its length.
low pressure
high pressure
Pressure (N/m2) = 10 N/kg x depth (m) x density (kg/m3)
The pull
of gravity
The greater the depth, the higher
the pressure
The denser the liquid, the heavier it is

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 carrying a special oil called hydraulic fluid.
Force
applied
here
Force
transferred
here
Pressure inside all parts of the
hydraulic system is the same

16. hydraulic fluid slave pistons foot pedal master piston
Hydraulic brake
All hydraulic brake systems (e.g. in a car) use a small master piston and a bigger slave piston.
hydraulic fluid
slave pistons
foot pedal
master 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 of the car.

17. Hydraulic 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:
pressure =
force applied
area of master piston
The pressure is transmitted to the slave pistons and so the
force exerted by the slave piston can be calculated using:
pressure =
force exerted
area of slave piston
force exerted = pressure x area of slave piston
The slave piston has a larger area than the master piston.
So, the force exerted by the slave pistons on the brakes is greater than the force exerted by the driver on the brake pedal.

19. Hydraulic brake – calculations
The master piston of a car has an area of 5 cm2.
1. If a force of 10N is applied to the master piston, calculate the pressure created in the brake pipes.
2. If the slave piston has an area of 50 cm2, calculate the force exerted on the brake disc.
Calculations:
1. At the master piston, p = f = 10 N = 2 N/cm2
a cm2
2. At the slave piston, f = p x a = 2 N/cm2 x 50 cm2 = 100 N
So, the force exerted on the brake disc is ten times greater than the original force applied to the master piston.

20. Hydraulics activity

21. 9L Pressure and Moments Pressure Pressure in liquids Moments
Contents
9L Pressure and Moments
Pressure
Pressure in liquids
Moments
Summary activities

22. Force and rotation pivot 5N
A force acting on an object can cause it to turn about a 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?
pivot 5N

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.
pivot
The turning effect of a force is called a moment.

24. distance from force to pivot
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
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. f x d Moment equation The moment of a force is given by the equation:
moment = force (N) x distance from pivot (cm or m)
moment f x d
Moments are measured in newton centimetres (Ncm) or newton metres (Nm).

28. Moment calculation pivot
Gina weighs 500 N and stands on one end of a see-saw. She is 0.5 m from the pivot.
What moment does she exert?
moment = 500 x 0.5
= 250 Nm
0.5 m
500 N
pivot

29. Principle of moments pivot The girl on the left exerts
an anticlockwise moment,
which equals...
The girl on the right exerts a clockwise moment, which equals...
her weight x her distance
from pivot
her weight x her distance
from pivot

30. Principle of moments pivot
If the anticlockwise moment and clockwise moment are equal then the see-saw is balanced. This is known as the principle of moments.
When something is balanced about a pivot:
total clockwise moment = total anticlockwise moment

31. Both moments are equal and so the see-saw is balanced.
Principle of moments
The principle of moments can be investigated using 10g masses with this balance.
moment (left) = 10 x 7
= 70 gcm
moment (right) = (10 x 3) + (10 x 4)
= 70 gcm
Both moments are equal and so the see-saw is balanced.

32. Principle of moments – calculation
Two girls are sitting on opposite sides of on a see-saw. One girl weighs 200 N and is 1.5 m from the pivot. Where must her 150 N friend sit if the see-saw is to balance?
When the see-saw is balanced:
total clockwise moment = total anticlockwise moment
200 N x 1.5 m = 150 N x distance
200 x 1.5 = distance
150
distance of second girl = 2 m

33. Why don’t cranes fall over?
Tower cranes are essential at any major construction site.
load arm
trolley
counterweight
loading platform
tower
Concrete counterweights are fitted to the crane’s short arm. Why are these needed for lifting heavy loads?

34. Why don’t cranes fall over?
Using the principle of moments, when is the crane balanced?
3 m
6 m
10,000 N ?
moment of = moment of
load counterweight
If a 10,000 N counterweight is three metres from the tower, what weight can be lifted when the loading platform is six metres from the tower?

35. Why don’t cranes fall over?
moment of
load =
= ? x 6
load x distance of load from tower
moment of
counterweight
distance of counterweight from tower =
= 10,000 x 3
= 30,000 Nm
counterweight x
moment of load = moment of counterweight
? x 6 = 30,000
? = 30,000 6
? = 5,000 N

36. Crane operator activity
At what distance can the loading platform carry each load safely?

37. 9L Pressure and Moments Pressure Pressure in liquids Moments
Contents
9L Pressure and Moments
Pressure
Pressure in liquids
Moments
Summary activities

38. counterbalance – A weight used to balance another weight.
Glossary
counterbalance – A weight used to balance another weight.
effort – The force applied to use 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 certain area. It equals the force divided by area and can be measured in pascals (Pa).

39. Anagrams

40. Pressure – true or false?

41. Match the definition

42. Multiple-choice quiz