AQA GCSE Physics

Revision Notes

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

drawing-pin, IGCSE & GCSE Physics revision notes

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

pressure-&-force, IGCSE & GCSE Physics revision notes

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 equation, downloadable AS & A Level Physics revision notes

  • 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:

Pressure triangle (2), IGCSE & GCSE Physics revision notes

  • 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

Pressure on different areas, downloadable AS & A Level Physics revision notes

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.

Pressure Worked Example GCSE, downloadable IGCSE & GCSE Physics revision notes

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

Pressure 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.
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