An iron needle sinks in water but a huge ship floats on the surface of water.
All objects whether inside the liquid (or fluid) or partially inside, appear to be lighter than in air. This is because liquids (or fluids) exert upward force on the immersed object. This is called the upthrust, the Buoyant force or buoyancy. The upthrust due to liquid pressure, pushes the object from all sides, but is greatest on the bottom where the liquid is deepest.
When a body is immersed wholly or partially in a liquid (or fluid) at rest, it appears to lose some of its weight. The apparent loss in weight of the body is equal to the weight of liquid displaced.
Consider a solid cylinder of height 'h' and area of cross-section A, to be completely immersed in a fluid of constant density r. The horizontal thrusts on the cylinder balance each other.
Consider the vertical thrust on the cylinder.
upthrust = weight of the displaced fluid.
