# 5.5.1 Pressure

### Pressure in a Fluid

• A fluid is either a liquid or a gas
• Pressure is defined as

The concentration of a force or the force per unit area

• For example, when a drawing pin is pushed downwards:
• It is pushed into the surface, rather than up towards the finger
• This is because the sharp point is more concentrated (a small area) creating a larger pressure

When you push a drawing pin, it goes into the surface (rather than your finger)

• Example 1: Tractors
• Tractors have large tyres
• This spreads the weight (force) of the tractor over a large area
• This reduces the pressure which prevents the heavy tractor from sinking into the mud
• Example 2: Nails
• Nails have sharp pointed ends with a very small area
• This concentrates the force, creating a large pressure over a small area
• This allows the nail to be hammered into a wall
• When an object is immersed in a liquid, the liquid will exert pressure, squeezing the object
• The pressure exerted on objects in fluids creates forces against surfaces
• These forces act at 90 degrees (at right angles) to the surface

The pressure of a fluid on an object creates a force normal (at right angles) to the surface

### Calculating Pressure

• The pressure at the surface of a fluid can be calculated using the equation:

• Pressure is measured in the units Pascals (Pa)
• The area should always be the cross-sectional area of the object
• This means the area where the force is at right angles to it
• This equation can be rearranged with the help of a formula triangle:

• This equation means:
• If a force is spread over a large area it will result in a small pressure
• If it is spread over a small area it will result in a large pressure

High heels produce a higher pressure on the ground because of their smaller area, compared to flat shoes

#### Worked Example

The diagram below shows the parts of the lifting machine used to move the platform up and down.

The pump creates pressure in the liquid of 5.28 × 105 Pa to move the platform upwards. Calculate the force that the liquid applies to the piston.

Step 1: List the known quantities

• Cross-sectional area = 2.73 × 10-2 m2
• Pressure = 5.28 × 105 Pa

Step 2: Write down the relevant equation

Step 3: Rearrange for the force, F

F = p × A

Step 4: Substitute the values into the equation

F = (5.28 × 105) × (2.73 × 10-2) = 14 414.4

Step 5: Round to the appropriate number of significant figures and quote the correct unit

F = 14 400 N = 14.4 kN (3 s.f)

#### Exam Tip

Look out for the units for the force!

Large pressures produce large forces – this is sometimes in kN! (1 kN = 1000 N)

### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
Close