Question

Why does a block of plastic released under water come up to the surface of water ?

Answer 1

According to Archimedes' principle

“The upward buoyant force that is exerted on a body immersed in a fluid, whether partially or fully submerged, is equal to the weight of the fluid that the body displaces and acts in the upward direction”.

Since the density of plastic is less than that of water, so the force of buoyancy(upward) on plastic block will be greater than the weight of plastic block(downward) . Hence, the plastic block will experience net force in upward direction, and comes up to the surface of water.