A pendulum is constructed as a light thin-walled sphere of radius [tex]R[/tex] filled up with water and suspended at the point [tex]O[/tex] from a light rigid rod. The distance between the point [tex]O[/tex] and the center of the sphere is equal to [tex]l[/tex]. How many times will the small oscillations of such a pendulum change after the water freezes? The viscosity of the water and the change in its volume on freezing are to be neglected. Why is the period of a solid pendulum different from that of a liquid one? Is it because the water shifts around inside the sphere? I need a hint.