1. IGCSE Physics
Pressure

2. Contents
Pressure
Pressure
Pressure in liquids

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. Contents
Pressure
Pressure
Pressure in liquids
Summary activities

11. 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.

12. 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.

13. 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

14. 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.

15. 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.

16. Hydraulic brake – calculations
The master piston of a car has an area of 5cm2.
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 50cm2 = 100 N
So, the force exerted on the brake disc is ten times greater than the original force applied to the master piston.

17. Hydraulics activity