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Pressure is force per unit area. Pressure determines the effect of a force on a surface.

Pressure exerted by different shoes

Different styles of shoe can cause different pressures due to their area.

• Flat shoes spread the force over a large area, reducing the pressure. Snow shoes have a much larger area than feet to spread the force over a larger area and reduce the pressure on the snow - this stops people sinking into the snow.
• High heeled shoes transfer the force through a much smaller area, causing a much greater pressure. It will hurt more if a person steps on someone’s foot in high heels than if they were wearing flat shoes. This is also why it hurts so much to accidentally step on a plug or stones barefoot - the force acts on a small area and the pressure is increased.

Calculating pressure in fluids

Liquids and gases are fluids. An object in a fluid experiences a pressure caused by both the fluid itself and the atmospheric pressure.

The pressure in fluids causes a force normal to a surface. A force that is normal to a surface acts at right angles (90°) to it. To calculate the pressure at the surface of a fluid use the equation:

This is when:

• pressure (P) is measured in pascals (Pa)
• force (F) is measured in newtons (N)
• area (A) is measured in metres squared (m²)

Example

A fluid exerts a force of 50 N over an area of 2 m². Calculate the pressure on the surface.