(i) Buoyancy and gravitational force are the two forces which act on a body, when it is immersed in a liquid.The conditions for a body required to float or sink in a liquid are as follows:
(a) If weight of the body is more than the upthrust act on it, then body will sink.
(b) If weight of the body is less than the upthrust act on it, then body will float.
(ii) Volume of the water displaced by an iron nail is less than that of piece of wood. That is why, more buoyancy force act on piece of wood and it floats on the surface of water.