Let's picture an empty sperical shell in inter galactic space or beyond. Inside this shell, a ball is rolling on the inner surface. Because of friction and the centrifugal force the ball exerts on the shell, the shell is acted on in two perpendicular direction. A radial and an angular direction.

I can't get a true grip on what happens. Will the ball eventually stop rolling, and the final situation be the sphere and ball rotating in concert around their center of mass?


1 Answer 1


Sounds like initially an external observer would see the shell not spinning, but oscillating around as the ball rolls inside.

Friction will couple the motion of the ball to the rotation of the ball.

Drag and losses will couple the motion of the ball to the motion of the sphere. The sphere's rotation will speed up and the ball's rotation will slow down. Eventually the ball will no longer be "rolling" inside the sphere (zero relative motion).

The static collection of the ball and the sphere will be rotating around the system's center of mass. That point will be on the ball side of the sphere's center. The sphere will appear to oscillate around the point as it did at the beginning, but now it will be spinning as well.


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