• Applications of the principle of Floatation

    (a) Ships:  Although it is made of iron and steel which are materials denser than water a ship floats in water. This is due to the fact that a floating ship displaces a weight of water equal to its own weight including that of the cargo. The volume of the ship is much larger than the volume of the material with which it is made. Since the empty space in the ship contains air, therefore, its average density is less than the density of water. Thus a ship floats with a small section under water.

    (b) Submarines: A submarine sinks by taking water into its buoyancy tanks. Once submerged, the upthrust is unchanged but the weight of the submarine increases with the inflow of water and it sinks faster. To surface, compressed air is used to blow the water out of the tanks. Each submarine is provided with ballast tanks. If the submarine has to submerge these tanks are filled with water. This makes the average density of the submarine greater than that of water as a result it sinks. When the submarine has to be surfaced, compressed air is blown into these tanks to expel the water. Again the average density of the submarine becomes less than that of water, hence it floats.

    (c) Balloons:  A balloon filled with hydrogen or hot air, weight less than the weight of air it displaces. The upthrust is, therefore, greater than its weight and resultant upward force on the balloon causes it to rise. Meteorological balloons, carrying scientific instruments called radiosondes are sent into the upper atmosphere. A small radio transmitter sends signals back to Earth which contains information about the temperature, pressure and humidity. They are racked by radar to give data on wind direction and speed.