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Two wheels with radii R1 and R2 (R1>R2) have rotational inertia I1 and I2, respectively. Initially the small wheel is at rest, while the big wheel has rotational velocity of W1 about its center. We gradually move the small wheel closer to the big one, until the wheels touch. Then friction between the wheels causes the small wheel to speed up, and the big wheel to slow down, until at last the two wheels spin with the same tangential velocity in opposite directions. What is this tangential velocity? (Hint: angular momentum is not conserved.)

I don't understand why angular momentum would not be conserved, considering that the frictional forces are of the same magnitude and opposite direction. Where does the external torque come from? Could someone enlighten me on this?