Pressure & Forces (Cambridge (CIE) O Level Physics): Revision Note

Pressure

• 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