Submarines

According to Archimedes’ principle, when a body is immersed in a liquid, it suffers an apparent loss of weight equal to the weight of the liquid displaced by it.

In the submarine boats, there is an internal water tank. When cruising the free surface level of water is AB and a considerable portion of boat is above the water.(a)

         

The boat may be sunk lower in the water by admitting water into the internal tanks. Now the free surface of water AB may be much above the boat (b). Pumps are used for emptying the water tanks, thus brining the boat again to its original surface level.