The motion of a pendulum is an example of mechanical energy conservation. There are two forces acted upon the pendulum. The first one is gravity. The force of the gravity acts in a descending direction and does work upon the bob of the pendulum. The other one is tension. Tension is an outer force that would help to change the total mechanical energy of the bob. Thus, the mechanical energy in the pendulum transforms to thermal energy as the pendulum slows down and eventually stops moving.
