Question

A piece of wood left under water comes to surface

Answer 1

Density of wood is less than the density of water. That is why it floats on the surface of water and if immersed, the upthrust acting on the wooden piece will be greater than its weight pulling it downwards, so it comes to the surface.

Answer 2

because the buoyant force works on the wood.density of wood is less than density of water,therefore it comes to the surface.