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

• To increase pressure - increase the force or reduce the area the force acts on. For example, to cut up food, a person can either press harder on the knife being used or use a sharper one (sharper knives have a smaller surface area on the cutting edge of the blade).
• To reduce pressure - decrease the force or increase the area the force acts on. For example, if a person was standing on a frozen lake and the ice started to crack, they could lie down to increase the area in contact with the ice. In this case, the ice would be experiencing the same force (the person’s weight), but it would be spread over a larger area, so the pressure would reduce. Snow shoes work in a similar way.

Calculating pressure

Liquids and gases are fluids. 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 is measured in pascals (Pa)
• force is measured in newtons (N)
• area 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.