call it oscillation (like, the oscillation of a branch of a tree), while when the frequency is high, we call it vibration (like, the vibration of a string of a musical instrument). Simple harmonic motion is the simplest form of oscillatory motion. This motion arises when the force on the oscillating body is directly proportional to its displacement from the mean position, which is also the equilibrium position. Further, at any point in its oscillation, this force is directed towards the mean position.
In practice, oscillating bodies eventually come to rest at their equilibrium positions because of the damping due to friction and other dissipative causes. However, they can be forced to remain oscillating by means of some external periodic agency. We discuss the phenomena of damped and forced oscillations later in the chapter. Any material medium can be pictured as a collection of a large number of coupled oscillators.
The collective oscillations of the constituents of a medium manifest themselves as waves. Examples of waves include water waves, seismic waves, electromagnetic waves. We shall study the wave phenomenon in the next chapter. .
. Period and frequency We have seen that any motion that repeats itself at regular intervals of time is called periodic motion . The smallest interval of time after which the motion is repeated is called its period . Let us denote the period by the symbol T .
Its SI unit is second. For periodic motions, which are either too fast or too slow on the scale of seconds, other convenient units of time are used. The period of vibrations of a quartz crystal is expressed in units of microseconds ( – s) abbreviated as µ s. On the other hand, the orbital period of the planet Mercury is earth days.
The Halley’s comet appears after every years. The reciprocal of T gives the number of repetitions that occur per unit time. This quantity is called the frequency of the periodic motion . It is represented by the symbol ν .
The relation between ν and T is ν = /T ( . )