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

An object floats or sinks when placed on the surface of water based on a principle called buoyancy, which is influenced by the object's density compared to the density of water. If the object's density is less than that of water, it displaces a volume of water whose weight is equal to or greater than the object's weight, causing it to float. Conversely, if the object's density is greater than water, it displaces less weight in water than its own weight, causing it to sink. This behavior follows Archimedes' principle, where the buoyant force acting upwards on the object is equal to the weight of the displaced water. An object floats when this buoyant force is equal to or greater than its weight, and sinks when the buoyant force is less.