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Class XI

PHYSICS

Ch- 14 - OSCILLATIONS

LESSON NOTES

• Periodic Motion
Motions, processes or phenomena, which repeat themselves at regular intervals, are called
periodic.
• Oscillatory Motion
The motion of a body is said to be oscillatory motion if it moves to and fro about a fixed point after
regular intervals of time. The fixed point about which the body oscillates is called mean position or
equilibrium position.
• Simple Harmonic Motion
Simple harmonic motion is a special type of periodic oscillatory motion in which
(i) The particle oscillates on a straight line
(ii) The acceleration of the particle is always directed towards a fixed point on the line.
(iii) The magnitude of acceleration is proportional to the displacement of the particle from the

• Characteristics of SHM
The displacement x in SHM at time t is given by
x = A sin (ωt+ Ф )
where the three constants A, ω and Ф characterize the SHM, i.e., they distinguish one SHM from
another. A SHM can also be described by a cosine function as follows:
x = A cos (ωt + δ)
• The displacement of an oscillating particle at any instant is equal to the change in its position
vector during that time. The maximum value of displacement in an oscillatory motion on either side
of its mean position is called “displacement amplitude” or “simple amplitude”.
Thus, amplitude A = x max.
• The time taken by an oscillating particle to complete one full oscillation to and fro about its mean
(equilibrium) position is called the “time period” of SHM. It is given by

• Frequency
The number of oscillations in one second is called frequency. It is expressed in sec-1 or Hertz.
Frequency and time period are independent of amplitude.

• Phase
The quantity (ωt+ Ф) is called the phase of SHM at time t; it describes the state of motion at that
instant. The quantity Ф is the phase at time f = 0 and is called the phase constant or initial phase
or epoch of the SHM. The phase constant is the time-independent term in the cosine or sine
function.