As an example, consider the vibration of the simple pendulum shown in Fig. 1.10. Let
the bob of mass m be released after being given an angular displacement At position 1
the velocity of the bob and hence its kinetic energy is zero. But it has a potential energy of
magnitude with respect to the datum position 2. Since the gravitational
force mg induces a torque about the point O, the bob starts swinging to the left
from position 1. This gives the bob certain angular acceleration in the clockwise direction,
and by the time it reaches position 2, all of its potential energy will be converted into
kinetic energy. Hence the bob will not stop in position 2 but will continue to swing to position
3. However, as it passes the mean position 2, a counterclockwise torque due to gravity
starts acting on the bob and causes the bob to decelerate. The velocity of the bob
reduces to zero at the left extreme position. By this time, all the kinetic energy of the bob
will be converted to potential energy. Again due to the gravity torque, the bob continues to
attain a counterclockwise velocity. Hence the bob starts swinging back with progressively
increasing velocity and passes the mean position again. This process keeps repeating, and
the pendulum will have oscillatory motion. However, in practice, the magnitude of oscillation
gradually decreases and the pendulum ultimately stops due to the resistance
(damping) offered by the surrounding medium (air). This means that