SUBMARINE: Submarine is a water-tight boat which can travel under water like a ship. It is providgd with water tanks. When the submarine is to dive, water is filled in water tanks and it is made heavier and the average density of submarine becomes greater than the density of seawater and it sinks. To make the submarine rise to the surface of the water, water tanks are emptied and the average density of the submarine becomes less than the density of seawater and it rises to the surface of the water.
[3 marks]