Work is done when a force acts on something, which moves in the direction in which the force is acting (page 110). a. When the bob swings back and forth, the string tension stays perpendicular to the direction of in which the bob is moving. There is no tension force component in the direction of motion; the work done by tension is zero. b. When the pendulum swings, the gravitational force does have a component parallel to (or opposite to) the direction of motion, and performs work. The work done by gravity serves to change the bob's KE. c. When the bob is at the very bottom, gravity does not act along the direction of the bob's motion. At that one position of the pendulum, gravity does no work.