Question

Why is it easier to swim in seawater than in river water?

Answer 1

Hint: Seawater has salt that increases its density while river water does not have salt. Archimedes’ principle states that when a body is partially or fully immersed in a fluid it experiences an upward buoyant force equal to the weight of the liquid displaced by it. The weight of the liquid displaced obviously depends upon the volume and density of the liquid displaced.
The formula used: $\text{Buoyant force on body = weight of the fluid displaced by body}$
$\text{Mass = Density}\times \text{Volume}$

Complete step by step answer:
According to Archimedes’ principle when a body is partially or fully immersed in a fluid it experiences an upward buoyant force equal to the weight of the liquid displaced by it.
 $\text{Buoyant force on body = weight of the fluid displaced by body}$
${{M}_{l}}g$
Where ${{M}_{l}}$ is the mass of the liquid displaced. Using the general formula
$\text{Mass = Density}\times \text{Volume}$,
We get the buoyant force as
$={{V}_{l}}{{\rho }_{l}}g$
Where ${{V}_{l}}$ and ${{\rho }_{l}}$ are the volume of the liquid displaced by the body and the density of the body respectively.
Thus, from the equation, we can see that the buoyant force provided by a fluid directly depends on the density of the liquid.
Now, since seawater has salt in it, the density of seawater is more than river water which is freshwater. Thus, the buoyant force offered by seawater to a body is also more than the river freshwater.
Thus, it is easier for a person to swim in seawater since he gets a bit more help from the buoyant force.

Note: There is another factor that comes into play when swimming, that is the viscosity of the fluid. The viscosity of the fluid offers resistance to relative motion between the layers of the fluid. Based on only the Archimedes’ principle, one may think that hypothetically it would be very easy to swim in honey since it has a higher density than water and would provide more buoyant force. However, honey is very viscous, so it would be very hard to swim in honey.