Below is an applet showing two harmonic oscillators of the same frequency. They could be made to vibrate either in phase or out of phase.

Related Physics principles:

(i)

Hooke's law: The tension in a spring is proportional to its extension from its natural length.

(ii)

Simple harmonic motion (SHM): An object exhibits this kind of oscillating motion when the acceleration of the object is (i) proportional to the displacement from equilibrium and (ii) always directed towards equilibrium. You see, the two objects "PHY" and "is phun", are now performing the simple harmonic motion.

(iii)

SHM and circular motion: SHM can be regarded as an axis-projection of a uniform circular motion.

(iv)

Phase difference: The phase difference between the two oscillators is the fraction of a circle by which one moves behind the other.