How much work is done by the string of a pendulum on its bob as it swings back and forth?

Question

answers:
a positive amount
zero
a negative amount
negative when going up and positive when going down

Answer 1

The work done by the string of a pendulum on its bob as it swings back and forth is zero.

This is because the tension in the string acts perpendicular to the direction of the bob's motion at all points in the swing. Since work is defined as the force component in the direction of the motion times the distance moved, and since the tension force acts perpendicular to the motion of the bob, the work done is zero.