Question

Why does a ball floating on water bobs up and down, if pushed down and released?

Answer 1

A ball floating on water is in equilibrium when its weight is balanced by the buoyant force.

If the ball is pushed down, it displaces more water, so the upthrust increases and becomes greater than its weight. This upward restoring force pushes the ball back toward its equilibrium position.

Due to inertia, the ball crosses the equilibrium position and continues moving upward. The process repeats, so the ball bobs up and down in oscillatory motion.