why is it easy to swim in sea water than in river water?
We know that the density of the sea water is more than the density of the river water. So, the weight of displaced sea water is more than the weight of displaced river water. This weight gives the upward Buoyant force to the swimmer. Thus, the apparent weight of the swimmer is less in the sea water. Hence he finds it easier to swim in the sea water.
when a person is floating on the water the its apparent weight is equal to,
= true weight of the person( Mg ) - weight of the liquid displaced( Vρg )
M is the mass of the person.
ρ is density of the water.
So, if the density water is more then the apparent weight of the person is less. therefore it easy to swim in the see water.
Sea water contains dissolved minerals like salt and other chemicals which make it denser than river water.
Since, buoyancy = density of water × volume of water displaced × acceleration due to gravity.
So, sea water provides higher buoyant force to the swimmer which makes it easier for the swimmer to swim in sea water.