I have an interesting question for you guys, I liked working on this one but I don't know if I gave the right answer, so here it is:

A Soft round exercise ball obviously will bounce up and down on th efloor, but whenever it leaves the floor it moves only undergravity. Consider instead small bounces, so that the ball jiggles up and down while staying in contact with the floor. For such small oscillations in height of the centre of mass, what is the restoring force? Is it a linear restoring force, and if so what oscillation frequency results? If it is not linear, outline what physics you are able, to describe the behaviour. Consider the ball is inflated to 1/20th of an atmosphere of pressure, and that is has a mass of M = 1kg.