Lesson 1 of 9 · 6 min
Periodic and oscillatory motion
NCERT §13.1, §13.2
On Dadaji's workbench a marble rolls in a brass bowl, up one side, down, up the other, again and again. Above it the ceiling fan turns round and round. Both repeat. Only one of them is an oscillation.
The story this chapter follows: Kavya at Dadaji's clock shop
The lesson in notes
In short
Any motion that comes back to the same state again and again, at equal time intervals, is periodic: a planet in its orbit, a ball bounced between palm and floor, an insect that climbs a ramp, drops off and starts again.
An oscillatory motion is a periodic motion to and fro about a mean position, like a swing, a cradle, the pendulum of a wall clock or a boat bobbing on a river.
At the mean position (the equilibrium position) no net force acts, so a body left there at rest stays there. Push it a little away and a force appears that pulls it back; that is what sets up the oscillation.
Every oscillatory motion is periodic, but a periodic motion need not be oscillatory. Uniform circular motion repeats itself but has no back-and-forth about a mean point.
Oscillation and vibration are the same thing. By habit a slow one is called an oscillation (a swaying branch) and a fast one a vibration (a sitar string).
The height-time graph of a ball bounced between hand and floor is built from pieces of parabolas, so the motion is periodic without being simple harmonic.
Simple harmonic motion (SHM) is the simplest oscillation: the force on the body is proportional to its displacement from the mean position and always points back towards that position.
Real oscillators come to rest at equilibrium because friction and other dissipative effects damp them; an external periodic push can keep them going.
A material medium behaves like a very large number of coupled oscillators, and their collective oscillation travels through it as a wave.