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

Hint: Apply Archimedes Principle. The liquid exerts a force when a body is immersed in it or placed on its surface. This resultant force is in upward direction. An object sinks in liquid if the density of an object is more than the density of liquid. This is because the weight of an object is more than buoyant force acting on the object.

Complete answer step by step:
If the density of the object is more than the density of liquid then the object sinks in the liquid and if the density of the is less than the density of liquid then the object floats on the surface of liquid. This happens because when a body is immersed in water a buoyant force is exerted by the liquid on that body. This was explained by Archimedes Principle which states that “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 at the centre of mass of the displaced fluid.” This upward buoyant force is called ‘Thrust’. When we enter a swimming pool or river for swimming we feel little weightlessness, the reason for this is the thrust exerted by the water.

Additional Information:
The weight of an object is the total force felt by it. In liquids, there are mainly two forces acting on, gravitational force and buoyant force (thrust). The apparent weight of an object is the difference of these two forces.
Apparent weight = Gravitational force – buoyant force (Thrust)

Note: Object floats on water if density of object is less than or equal to the density of the liquid. This is because the buoyant force acts on the weight of the object. Our weight depends on the fluid surrounding us. We feel different weights in lake, river, etc., where we are surrounded by water than on the ground (surrounded by air). The buoyant force is responsible for this apparent weight.