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Periodic and oscillatory motions

  • Periodic motion: A motion which repeats itself after a fixed interval of time

  • Examples:

  • Motion of the moon around the earth

  • Motion of the hands of a clock

  • Oscillatory motion: A body in oscillatory motion moves to and fro about its mean position in a fixed time interval.


  • Motion of the pendulum of a wall clock

  • Motion of the liquid contained in a U-tube when one of its limbs is compressed.

  • Period (T): It is the interval of time after which a motion is repeated. Its unit is seconds (s).

  • Frequency (ν): Number of repetitions that occur per unit time

Its unit is (second)-1 or Hertz.

  • Displacement: Change in position

The figure shows a block attached to a spring.

Here, displacement is x.

An oscillating simple pendulum’s angular displacement isβ.

  • Displacement variable may take negative values.

  • Periodic functions can be expressed as a superposition of the sine and cosine functions.

  • Oscillations of a block of mass, m fixed to a spring, which is in turn fixed to a rigid wall, are shown in the figure.

  • The block is pulled and released so that it executes to and fro motion (SHM).


m = Mass of the block

+A, −A = Maximum displacement

(x = 0) = Position of the centre of the block at the equilibrium of the spring

  • When the block is pushed to the right side, one spring is compressed while the other is elongated hence the block is subjected to a restoring force of F (x), which is proportional to the displacement, x (in the opposite direction).

As the block feels twice of restoring force because of two spring system,

∴ F

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