Question

one end of a rod of negligible mass is attached to a block and the other end is attached to a pivot. initially, the block is held in place with the rod horizontal. the block is then released and swings down through its lowest point, as shown, before swinging back up again. frictional forces are negligible. which of the following statements correctly indicate the work done on the block while it is swinging down? a work is done only by the tension force from the rod. b work is done only by the gravitational force. c work is done by a combination of the tension force and the gravitational force. d no work is done on the block.

Explanation:

Work done by a force is defined as $W = F \cdot d \cdot \cos\theta$, where $\theta$ is the angle between the force and displacement. For the tension force from the rod, it always acts perpendicular to the block's circular path ($\theta = 90^\circ$, $\cos90^\circ=0$), so no work is done by tension. The gravitational force acts downward, and the block has a downward component of displacement as it swings down, so $\cos\theta
eq 0$ and work is done by gravity.

Answer:

B. Work is done only by the gravitational force.