Question

A man first swims in sea water and then in river water. (i) Compare the weights of sea water and river water displaced by him.

(ii) Where does he find it easier to swim and why? 

Answer 1

(i) 1:1; The weight of the water displaced by the man in sea and river will be same and will be equal to his own weight.

(ii) He finds it easier to swim in the sea because the density of sea water is more than the density of river water. So his weight is balanced in sea water with a part of his body submerged in the water.

Answer 2

(i) When a body is completely immersed in water then it displaces an equal volume of water to its own weight. The volume of the body of man is the same in both river and sea so the weight of the water of the sea displaced by him is equal to the weight of the water of the river displaced by him. And the ratio of weights would be 1:1.
(ii) Sea water contains mineral salts and the density of sea water increases due to the presence of these. As the density of sea water is more than the normal water so it applies more buoyant force than the usual one and a person finds it easy to swim in sea water.