Kinetic and Potential Energy
The total energy of some isolated system remains the same. However, that energy can be present in different forms. For example, kinetic energy can become potential energy.
The simple pendulum is a good example
of how potentia energy and kinetic energy interchange, even though the
total energy always remains the same. The animation below shows that
the pendulum has its maximum kinetic energy when it is at the bottom
of its swing - this is when it is moving the fastest. When it reaches
the "top" on either side (the point where it comes to an instantaneous
stop and turns around), the pendulum has only potential energy. The
total energy of the pendulum is the sum of the kinetic energy and the
potential energy. The total energy is constant, but the proportions
of kinetic and potential energies change.