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

If the density of an object is more than the density of the liquid, then it sinks in the liquid. This is because the buoyant force acting on the object is less than the force of gravity. On the other hand, if the density of the object is less than the density of the liquid, then it floats on the surface of the liquid. This is because the buoyant force acting on the object is greater than the force of gravity

If the objects of density less than that of a liquid float on the liquid. The objects of density greater than that of a liquid sink in the liquid.

`to` An object sink in water if its density is greater than that of water .
`to` An object floats in water if its density Is less than that of water .

When an object is put in water, then two forces act on it:
(i) weight of the object acting downwards (which tends to pull down the object), and
(ii) buoyant force (or upthrust) acting upwards (which tends to push up the object)
Now, whether an object will float or sink in water will depend on the relative magnitudes of these two forces (weight and buoyant force) acting on the object in opposite directions.
If the buoyant force (or upthrust) exerted by water is equal to or greater than the weight of the object, will float in water.
If the buoyant force (or upthrust) exerted by water is less than the weight of the object,will sink.