Pressure in Fluids

All liquids and gases are fluids. A solid exerts pressure on a surface due to its weight. Similarly, fluids have weight, and they also exert pressure on the base and walls of the container in which they are enclosed. Pressure exerted in any confined mass of fluid is transmitted undiminished in all directions.

Buoyancy

The buoyant force is a force that is exerted on an object which is partially or completely immersed in a fluid. The other name for this upward force is called upthrust. Because of the presence of buoyant force, a body that is submerged fully or partly in a fluid will lose its weight.

The magnitude of this buoyant force depends on the density of the fluid.

Why objects float or sink when placed on the surface of water?

The nail sinks. The force due to the gravitational attraction of the earth on the iron nail pulls it downwards. There is an upthrust of water on the nail, which pushes it upwards. But the downward force acting on the nail is greater than the upthrust of water on the nail. So it sinks.

The cork floats while the nails sinks. This happens because of the difference in their densities. The density of a substance is defined as the mass per unit volume. The density of cork is less than the density of water. This means that the upthrust of water on the cork is greater than the weight of the cork. So it floats.

Fig:- An iron nail sinks and a cork floats when placed on the surface of water.