Pressure

O Level / IGCSE Physics · Motion, forces and energy. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Pressure tells you how concentrated a force is. The same force on a smaller area gives a larger pressure, which is why a sharp knife cuts well and wide tyres do not sink into soft ground. Pressure on a surface always pushes at right angles to that surface.

In a liquid, pressure increases with depth and with the density of the liquid. It does not depend on the shape or width of the container. At the same depth in the same liquid the pressure is the same.

The air also presses on everything. A liquid barometer measures this atmospheric pressure: the air holds up a column of liquid, and the vertical height of the column shows how large the pressure is.

Rules to remember

Common mistake

Using an area in cm2 or a depth in cm and still writing the answer in Pa. Change the area to m2 and the depth to m before you calculate.

Worked example

A swimmer dives to a depth of 4.0 m in a lake. The density of the water is 1000 kg/m3, g = 9.8 N/kg and atmospheric pressure is 100 000 Pa. What is the total pressure on the swimmer?

  1. Pressure due to the water: Δp = ρgΔh = 1000 × 9.8 × 4.0 = 39 200 Pa
  2. The atmosphere presses on the surface of the lake, so add it: 100 000 + 39 200
  3. Total pressure = 139 200 Pa

Answer: 139 200 Pa (about 1.4 × 105 Pa).

Practice questions

Try each one, then open the answer.

1. A plastic bottle full of water has small holes made all around its side at the same height. In which direction does the water leave each hole, just as it comes out?

  1. A
    at right angles to the side of the bottle
  2. B
    vertically downwards
  3. C
    parallel to the side of the bottle, downwards
  4. D
    upwards, towards the top of the bottle
Show answer

Answer: A. The pressure of the water produces a force at right angles to every surface it touches, so each jet starts out perpendicular to the wall before gravity curves it downwards.

2. What is the increase in pressure at a depth of 5.0 m in water of density 1000 kg/m3? (g = 10 N/kg)

  1. A
    500 Pa
  2. B
    5 000 Pa
  3. C
    50 000 Pa
  4. D
    500 000 Pa
Show answer

Answer: C. Δp = ρgΔh = 1000 × 10 × 5.0 = 50 000 Pa.

3. A box of weight 400 N rests on the floor on a face of area 0.50 m2. What pressure does it exert on the floor?

  1. A
    200 Pa
  2. B
    400 Pa
  3. C
    800 Pa
  4. D
    2000 Pa
Show answer

Answer: C. pressure = force ÷ area = 400 ÷ 0.50 = 800 Pa.

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