Pressure & Forces (Cambridge (CIE) IGCSE Physics): Revision Note

Pressure

What is pressure?

• Pressure is defined as

The force per unit area

• Pressure is measured in newtons per metre squared (N/m²)

  • 1 newton per metre squared is equivalent to 1 pascal (Pa)

  • 1 N/m² = 1 Pa

• It can be calculated using the following pressure equation:

$\mathrm{pressure}=\frac{\mathrm{force}}{\mathrm{area}}$

$p=\frac{F}{A}$

• Where:

  • p = pressure, measured in newtons per metre squared (N/m²) or pascals (Pa)

  • F = force, measured in newtons (N)

  • A = area, measured in metres squared (m²)

• 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

• If an object rests on more than one point of contact, add their areas together to get the total area the force is spread over

Examples of applications of pressure

• Example 1: Tractors

  • Tractors have large tyres

  • This spreads the weight (force) of the tractor over a large area

  • The weight of the tractor is unchanged, so this reduces the pressure and prevents the heavy tractor from sinking into the mud

• Example 2: Drawing pins

  • Drawing pins have sharp pointed ends with a very small area

  • This concentrates the force, creating a large pressure over a small area

  • This allows the drawing pin to be pushed into a wall

Applying a force to a drawing pin

When you push a drawing pin, it goes into the surface (rather than your finger)

• Example 3: High heels

  • High heels have small, sharp points with a small area

  • This concentrates the weight (force), creating a large pressure over a small area

  • Flat shoes have a larger area which the weight (force) is spread over resulting in a lower pressure

  • If the person's weight is the same in both shoes, this explains why high heels sink into soft surfaces more easily than flat shoes

The effect of surface area on pressure

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