Explain why it is easier to lift a stone under water than in air.

What is a submarine? How can it be made to dive in water and come to the surface of water?

The density of air is 1.28 g litre^-1. Express it in:

(a) g cm^-3

(b) kg m^-3

The dimensions of a hall are 10 m x 7 m x 5 m. If the density of air is 1.11 kg m^-3, find the mass of air in the hall.