CBSE Class 9 Answered

The weight of the displaced fluid is directly proportional to the volume of the displaced fluid (specifically if the surrounding fluid is of uniform density). Thus, among objects with equal masses, the one with greater volume has greater buoyancy.i.e.An object is buoyed up with a force equal to the weight of the liquid it displaces.

Now,the ship displaces amount of water equal to it's own weight thus doesn't sink.While the nail is not able to dispace sufficient volume of water due to it's shape and sinks.