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

To solve the question step by step, we will break it down into the two parts as presented.

### Step-by-Step Solution:

**(i) Compare the weights of seawater and river water displaced by him.**

1. **Understanding Displacement**: When the man swims, he displaces a volume of water equal to the volume of his body submerged in the water. The weight of the water displaced can be calculated using the formula:
\[ \text{Weight of displaced water} = \text{Volume displaced} \times \text{Density of water} \]

2. **Relative Densities**: The density of seawater is higher than that of river water due to the presence of salt. However, the question asks for a comparison of the weights of water displaced, not the densities themselves.

3. **Using Archimedes’ Principle**: According to Archimedes' principle, the weight of the water displaced by an object is equal to the weight of the object itself when floating. Since the man has a certain density, he will displace a volume of water that corresponds to his own weight.

4. **Conclusion for Part (i)**: Since the man displaces the same volume of water regardless of whether he is in seawater or river water, the ratio of the weights of seawater and river water displaced by him is:
\[ \text{Weight of seawater displaced} : \text{Weight of river water displaced} = 1 : 1 \]

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

1. **Density Comparison**: The density of seawater is greater than that of river water. This is primarily because seawater contains dissolved salts, which increase its density.

2. **Buoyant Force**: The buoyant force (upthrust) experienced by the man while swimming is directly proportional to the density of the fluid. Therefore, since the density of seawater is greater, the buoyant force in seawater will also be greater than in river water.

3. **Conclusion for Part (ii)**: The man will find it easier to swim in seawater because the greater buoyant force in seawater helps him stay afloat more easily compared to river water.

### Final Answers:
- (i) The weights of seawater and river water displaced by him are in the ratio of 1:1.
- (ii) He finds it easier to swim in seawater because the density of seawater is greater, resulting in a higher buoyant force.