This illustration and graph describe how and when energy transfers on a simple pendulum.
As the washer swings down, potential energy is transformed into kinetic energy. At the bottom of the swing all energy is kinetic energy. As the washer moves up, kinetic energy is transformed into potential energy. At its highest point in the arc of the swing, once again the energy is all potential energy.
When your partner moved the washer to the left or right from its resting position, he or she gave the pendulum potential energy. It was potential energy because the washer could swing due to its relative position and gravity. This kind of potential energy is known as gravitational potential energy.
What’s interesting about a pendulum, though, is that when you let go of it, the potential energy gradually transforms into kinetic energy. And then it gets faster and faster until it reaches the lowest point of the swing. At that point the kinetic energy starts transforming back into potential energy on the other end of the swing. Then, the pendulum actually comes to a stop! It comes to a stop for a very short time at the end of each swing. When it is “at rest” the energy is once again potential energy.
With this great system going, you might think the pendulum would never stop. But as you observed, it does indeed stop swinging after a while. But why? Where did all that energy go?