9.3 Torque Torque Puzzle

 

Explanation: As discussed in Section 9.3, the magnitude of the torque is equal the product of the magnitudes of the forces on the lever arm. The direction of the torque depends on the direction it tends to rotate the object. You can identify the direction of rotation by examining the relationship of the line of action of the force to the axis of rotation. A judicious choice of the axis of rotation makes the task much easier. For instance, this problem can be easily understood if we consider torques relative to the point of contact with the ground (instead of center of mass). Changing the slope of the string results in lines of action at different orientations relative to the axis of rotation, this in turn results in different directions of rotation. There is even a string inclination that will result in no rotation.

Source: Fu-Kwun Hwang