Two Types of Energy
The energy an object has due to motion is called kinetic energy. A moving object can do work when it strikes an object. When a baseball is thrown by a pitcher, the ball has kinetic energy.
When an object isn't moving but still contains stored energy due to its position or shape it has potential energy. When a book is held in the air it has the potential to do work. If the person holding the book lets go, the book will fall to the ground.
At position A, a pendulum that is swinging from side to side has all of its energy in the form of potential energy. As the pendulum swings to position B, its energy is converted into kinetic energy. Once the pendulum crosses the midline and begins swinging towards position C, energy is being converted back into potential energy and the cycle continues.