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

Hey samadhan, An object may float or sink when placed on the surface of water because of the principle of buoyancy. Which is quite simply the upward force that is exerted on an object by the fluid on whose surface the object is placed. Let's take a very simple example of a paper boat placed on a water. The boat at all times experiences a force that is upward in direction from the water. This force is exerted because the water is resisting the weight of the boat. This force increases as we go deep in to the water. That is why it is highly unlikely that a paper boat will not sink near the bottom of the water body, because its weight is not able to resist the higher force at the bottom, but it can resist the weaker force at the top. Any object whose density is greater than the fluid in which it is placed, will sink. That is why a heavy boat made of metal will sink when placed in a small tumbler of water, because the weight of the boat is more than the buoyant force that the water is exerted in this case. However, a big ship made of metal is afloat in a sea because the buoyant force in this case is balanced by the weight of the ship. The shape of the object also places an important role and a boat also because of its aerodynamics manages to stay afloat on water. The archimedes principle explains this very succinctly. "Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object." read less

the property of an object to float on water is called buoyancy Buoyancy is dependent on the difference between the density of water and the object. (Density is ratio of mass to volume.) if the density of an object is greater than water ( density of water is 1) eg.iron = 7 ,will not float if the density of an object is less than water eg. wood=0.7 ,it will float on water read less