Where is the centripetal force in this device?

Question

Lets say I have a 6” metal ring with a free wheeling ball bearing (bb) (1” in diameter, about 0.03kg) in it and a 3” rotor arm attached to a motor at the center of the ring. As the rotor spins it pushes the bb around and the cf forces pushes the bb to the wall of the ring. I rotate the arm at “100” rpm, bb goes around at 100 rpm.

the bb exerts a cf force on the inner face of the ring by running over it. however, is see no cp force acting on this system. The rotor arm pushes the bb around – therefore, always pushing a constant weight of 0.03kg around - however, with increased rpm the bb produces an increased cf force against the ring but there is still no cp force present or acting on the bb? Am I missing something with the rotor arm?

Answer 1

The arm is not causing the circular motion of the ball bearing. The wall of the device is providing the centripetal force necessary to keep the ball bearing in orbit. The only function of the arm is to apply a *tangential* force which speeds up or slows down the ball. But it cannot provide centripetal force (it is pushing tangentially). The wall provides the centripetal force to the ball bearing.

You knew this already, since you stated that the ball bearing exerts an outwards force on the wall. For every action, there is an equal and opposite reaction.