Why do things bob up and down on the water?

Answer 1

When an object is floating in the water there are two forces acting on it: the downward force of gravity and the upward force of buoyancy from the water. When it is floating still the forces are perfectly balanced and cancel each other out so the object does not move up or down. This kind of situation is called equilibrium. While the force from gravity on the object remains the same, the force from buoyancy depends on how much if it is underwater. If something pushes it down then buoyancy will by stronger than gravity, and the object will get pushed back up. In doing so the object will gain momentum and overshoot the equilibrium point as it goes up. The buoyancy force is now weaker than gravity, so the object falls back down, but overshoots the equilibrium again as it goes down. The cycle repeats as the object goes up and down until friction slows it to a stop.