Pressure & Forces (Cambridge O Level Physics)

• Pressure is defined as

The force applied 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

Equal Forces but Inequal Pressures

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

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

• Where:
  • P is pressure (Pa)
  • F is force (N)
  • A is area (m²)

• 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:

Formula Triangle for Pressure

Pressure, force, area formula triangle

• This equation tells us that:
  • 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 of High Heels vs Flat Shoes

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