# what is difference between oscillation and periodic motion

valentin68 | Certified Educator

There are two main differences between an oscillation and a periodic motion. The first difference is that an oscillation happens in a single direction (or in a plane) while periodic motion is generally described in 2 dimensional (or 3 dimensional) coordinates. For example a string oscillate only in one direction, while a car moves periodically in a closed circle. Thus one can say that an oscillation is the projection of a 2D periodic motion an a single coordinate axis. In fact this is why the `sin(..)` function appear in the expression of the oscillator elongation (because it is a projection).

There is also a second difference between an oscillation and a periodic motion. Unlike the first difference described above which is usually overlooked this is a major difference. An oscillation can be periodic or not. The classical example for a non-periodic oscillation is a damped oscillation. Its amplitude decreases over time, until the oscillation stops, thus the motion is not periodic. Only the oscillations that happens in a nondisipative medium (a medium where oscillator energy is not lost into heat, by friction for example) are periodic. These periodic oscillations are said to be undamped.