The topic of Simple Harmonic Motion gives us the tools to describe the motion of the frictionless mass-spring and the simple pendulum. Oscillatory motion occurs with these systems because when they are displaced from equilibrium, a force is produced proportional to the displacement from equilibrium. This 'restoring force' acts towards the equilibrium position of the system. However, when the system returns to its equilibrium point, it retains the kinetic energy associated with the work performed by the restoring force, and the mass-spring or the pendulum bob keeps going past equilibrium, which leads to restoring force from the other direction. Each cycle repeats. The mass-spring or pendulum oscillates.

The mass-spring and the simple pendulum are classic problems of Physics 101. There is no escaping them. You must go into the MCAT with a clear understanding of the behavior of these devices. However, the MCAT rarely presents you with the straight-forward model, being more apt to throw you something of a curve ball, such as a compound pendulum or a damped mass-spring. Never panic if the MCAT presents you with a complicated version of a simpler device you have learned. One of the main goals of MCAT preparation is to be prepared to apply fundamental principles within seemingly difficult contexts.