Pressure in solids and liquids — GCSE Physics practice questions

Question 1

A wardrobe of mass m stands on a bedroom floor.
The total area of its feet in contact with the floor is A, and the gravitational field strength is g.

Which expression gives the pressure that the wardrobe exerts on the floor?

Accepted Answer

D: m g / A. Pressure is force per unit area. The force pressing on the floor is the wardrobe's weight, W = m g, so p = m g / A.

Question 2

A seafood restaurant keeps live lobsters in a tank of sea water of density 1030 kg / m³.
The sea water in the tank is 38 cm deep. The gravitational field strength g is 9.8 N / kg.

What is the pressure due to the sea water at the bottom of the tank?

Accepted Answer

C: 3.8 kPa. p = ρ g h = 1030 × 9.8 × 0.38 = 3 836 Pa ≈ 3.8 kPa. The depth must be in metres for the answer to come out in pascals.

Question 3

A free-diver's wrist gauge shows the total pressure on it. At a depth of 12 m in a lake, the gauge shows 2.2 × 10⁵ Pa.
The atmospheric pressure at the surface of the lake is 1.0 × 10⁵ Pa. The diver goes deeper, until the gauge shows 3.4 × 10⁵ Pa.

What is the new depth of the diver?

Accepted Answer

C: 24 m. Subtract the atmosphere: the water gives 2.2 × 10⁵ − 1.0 × 10⁵ = 1.2 × 10⁵ Pa at 12 m, so 1.0 × 10⁴ Pa per metre. At the new depth the water gives 3.4 × 10⁵ − 1.0 × 10⁵ = 2.4 × 10⁵ Pa, which needs 24 m.