Pressure (AQA GCSE Physics)

• 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

• 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

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)