Pushing a floater under
What happens if you push a floating object right under with your hand? This small experiment reveals something important.
The experiment
Apply a vertical downward force to object A, which floats partly immersed, until it is completely under. Then take your hand away.
The observation
While you hold it under you feel an extra upward force against your hand.
When you take your hand away, the object returns to where it started.
Why this happens
Pushed fully under, the object displaces a larger volume of water. The upthrust therefore grows, and it is now greater than the weight of the object. That leaves a resultant force acting upwards — and that is what your hand feels.
Back to equilibrium
Release it and that resultant drives the object upwards. As it rises the immersed part shrinks and the upthrust falls, and it comes to rest at the point where the upthrust equals the weight of the object.
Equilibrium of a floating object
An object floats at the level where the upthrust from its immersed part is exactly equal to its weight. Push it deeper and the upthrust grows, lifting it back.