Pressure (AQA GCSE Physics)

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