If an object sinks in water, it means that

Answer Details

Whether a body floats or sinks is decided by comparing its weight with the maximum upthrust available. By Archimedes' principle the upthrust equals the weight of fluid displaced. When a body is fully submerged it displaces its own volume \(V\) of water, so

The body sinks when \(W > U_{\max}\), that is when \(\rho_b V g > \rho_w V g\). The common volume \(V\) and \(g\) cancel, leaving the condition \(\rho_b > \rho_w\). An object sinks in water precisely because its density is greater than the density of water.

This is why a small steel nail sinks while a large wooden log floats: what matters is density, not size or weight on its own. A steel ship floats only because its hull encloses air, which lowers the average density of the whole ship below that of water.

The statement that upthrust equals weight describes a body in equilibrium, which is the condition for floating or for remaining suspended at rest in the fluid, not for sinking; a sinking body has an upthrust smaller than its weight and so has a net downward force. Saying the density is less than that of water gives the condition for floating, the exact opposite. Comparing water pressure with weight is meaningless because pressure and force are different quantities with different units, so they can never be equated. In the examination, reduce every flotation question to a comparison of two densities, and check the units of any quantities you are asked to compare.