In Example 10.10 (Section 10.6) the angular speed v changes, and this must mean that there is nonzero angular acceleration. But there is no torque about the rotation axis if the forces the professor applies to the weights are directly, radially inward. Then, by Eq. (10.7), α_zmust be zero. Explain what is wrong with this reasoning that leads to this apparent contradiction.

Eq.(10.7)

Rotational analog of Newton's second law for a rigid body: Moment of inertia of rigid body about z-axis Angular acceleration of rigid body about z-axis Net torque on a rigid body about z-axis - (10.7) %3D ....