Question

In the classroom demonstration illustrated in V Figure 8.30, a person on a rotating stool holds a rotating bicycle wheel by handles attached to the wheel. When the wheel is held horizontally, she rotates one way (clock-wise as viewed from above). When the wheel is turned over, she rotates in the opposite direction. Explain why this occurs. [Hint: Consider angular momentum vectors.]

Answer 1

People are familiar with F = M a in rectilinear motion

Γ = I α is the corresponding relation in rotational motion.

Also, Γ = R X F gives the direction of the torque

When the wheel is turned over the direction of R is reversed since if it was pointing to the right the lever arm on which the force was acting is now pointing to the left. The gravitational force is still pointing downward so the direction of Γ has been reversed and the gravitational force is still in the downwards direction and there is a reversal in the torque acting on the wheel.

One can also think of the wheel having bendable spokes. A force that would turn the wheel and bend the spokes in one direction would have the opposite effect when the wheel was turned over. (A force forcing the spokes backwards would still force the spokes backwards when the wheel is turned over but now the wheel is rotating in the opposite direction)