Question

why does an object float or sink when placed on the surface of water

Answer 1

The upward force experienced by a body when partially or wholly immersed in a fluid is called upthrust or buoyant force. A body sinks in a liquid if its density is more than that of the liquid. ... If the buoyant force is less than the weight of the object in fluid the object sinks into the fluid completely.

Answer 2

an object will float on the water when its density is less than the density of water
an aboject will sink in water when its density is more than the water