physics notes

The law of floatation explains that, "A floating body displaces its own weight of the fluid in which it floats".

When a block of wood is placed in water it sinks until the weight of water displaced is just equal to its own weight. When this happens the block floats. Archimedes's principle and law of floatation can explain several phenomenons.

An iron nail sinks in water whereas a ship made up of iron and steel floats. This is due to the fact that a ship is hollow and contains air and therefore, its density is less than that of water.

A ship sinks in water to a level such that the weight of the displaced water equals its own weight. Since the density of the sea water is more than that of river water, a ship sinks less in sea water. It is for this reason that a ship rises a little when it enters a sea from a river.

It is because of the higher density of sea water that it is easier to swim in the sea.