Question

what do you mean by buoyancy ? why does an object float or sink when placed on surface of liquid

Answer 1

Buoyancy is the upward force experienced by an object when it is immersed in a fluid. 
(i) Its weight which acts vertically downwards, and
(ii) the buoyant force which acts vertically upwards.
When the weight is more than the buoyant force, the object sinks while it floats when the buoyant force is more than the weight of the object.

Answer 2

the force acts when any body or object is dipped into the water after that a force acts on that body and push it upwards that force is called buoyancy

if the mass of the object is lower than the water then it floats and vice versa