An object float or sink when placed on the surface of the water because,
- An object sinks in water if its density is greater than that of water.
- An object floats in water if its density is less than that of water.
- When the object’s weight force is balanced by the upward push of the water on the object.
- If the weight force down is larger than the water’s upward push on the object, then the object will sink.
The object floats or sinks due to the buoyant force. When an object is immersed in a liquid it experiences an upward force known as Buoyant Force. The buoyant force (or upthrust) pulling upwards (which tends to push up the object). Now, whether an object will float or sink in water will depend on the corresponding magnitudes of these two forces (weight and buoyant force) acting on the object in opposite directions.