What do a rocking chair, a bouncing ball, a vibrating tuning fork, a swing, and the Earth's orbit all have in common? They are all undergoing periodic motion. Periodic motion is defined as any repetitive motion that happens in equal intervals of time. There are many types of periodic motion, but one type that we will discuss is called Simple Harmonic Motion (SHM). Simple harmonic motion is specifically any type of periodic motion that moves back and forth across an equilibrium point in which a restoring force is proportional to the displacement from that equilibrium point.
Let's imagine a block attached to a horizontal spring that is at rest on a table with no friction as shown. What would happen if you were to pull this object to the right an inch or two and let go? It would oscillate—or move back and forth through an equilibrium point. The block, before being set in motion, was resting at that equilibrium point—the point where the force was zero. Every other point along the path of the back and forth motion, the spring would be pushing or pulling that block back toward the equilibrium point. The spring is providing the restoring force needed to keep that block in motion. We call it restoring force in physics because it acts to pull the object back to the equilibrium.
Question
If the mass-spring system is considered to be in simple harmonic motion, what is true about the forces involved?
The force is constantly changing as it is directly proportional to the displacement from the equilibrium point.
