An object will float on the surface of water when it has density less than that of water because it always displaces more weight of water than its own weight, and as the buoyant force is more than its own weight, therefore it floats. On the other hand, an object sinks in water when it has density more than that of water because it always displaces less weight of water than its own weight, and as the buoyant force is less than its own weight, it sinks in water.