Differentiate between Harmonic motion and Simple Harmonic motion. Is periodic and oscillatory motions are also periodic motion?

Harmonic motion is an oscillatory motion which can be expresed in terms of single harmonic function(sine function or cosine function). In such oscillations when a body is displaced from its equilibruim position, it will oscillate to and fro from its mean position under a restoring force, which is always directed towards the mean position and whose magnitude at any instant is proportional to the displacement of the body from the mean position at that instant.

Simple harmonic motion is a harmonic oscillation of constant amplitude and of single frequency. Harmonic oscillations possesses constant amplitude in the absence of frictional forces.Such a harmonic motion of constant amplitude and of single frequency is called simple harmonic motion. Simple harmonic motion is a special type of periodic motion.

All oscillatory motions are periodic because each oscillations gets completed in a definite interval of time. On the other hand all periodic motions may not be oscillatory. Eg: Revolution of planets around sun is a periodic motion but not an oscillatory motion(i.e no to and from motion about mean position is present).