Taking Advantage of Density

As we already know, the floatability of an object depends of the hydrostatic push of the fluid in which it is immersed. Objects which density is lower or equal to the density of the fluid will float.

Float in the water

Sometimes you would wonder, why all those big ships made of steel can float in the water? Even though the iron and steel are like 8 times denser than water, a boat can float because in its design, the major part of its volume it’s occupied by air, which is like nine hundred times less dense than water. Therefore the ship and the air contained inside of it, is less dense than water.

Submarines

The submarines obey the same principle we have applied to ships, but they are able to increase their density to submerge and navigate under water, and decrease it when they want to float and sail in the surface.

The increase or decrease of the density of a submarine is accomplished by pumping water from and into the ship. The submarines have, distributed along its structure, tanks that are filled with water to increase the density of the ship to submerge, or air to make it float.

 Up: the submarine in the surface; Middle: as the tanks fill the submarine submerge; Down: emptying the tanks makes the submarine go up again.

Fish Swim bladder

Most of Fish balance their density with the water surrounding them with a special organ, called the Natatory Bladder or Swim Bladder, located in its abdomen, and in some cases connected to the digestive system. This Bladder is a bag of muscle tissue filled with a mix of gases.

This Bladder may assist the fish increasing or decreasing its density. By compressing the air which is contained in it, the fish descends in the water; and it rises again by relaxing these muscles which had served to compress the bladder. The gas density is less than the density of water.

Source: snhs-plin.barry.edu/Research/Fish_Anatomy.htm