© Daniel Kleppner.
The bottom part of this illustration shows a harmonic oscillator: a mass attached to a spring that is attached to an immovable wall. If the mass is pulled to position -a and released, it will oscillate between -a and a. The diagram at the top of the illustration shows the total energy, E, kinetic energy, K, and potential energy, V, of the system as a function of position. It reveals that the motion is bounded and shows how the amplitude of the motion varies as the total energy is increased. Diagrams such as this can be invaluable in understanding more complex systems; for instance, two atoms that attract at long distances and repel at short distances. (Unit: 5)