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Sci Tech
Ball and pendulum
V. GANESAN
If a pendulum can swing to and fro endlessly, why does a ball thrown from a height not keep bouncing the same way?
R. Kumar
Madurai, Tamil Nadu
Any moving body continues to move unless it is opposed by a force like that of friction. A pendulum, in principle, should perpetually oscillate to and fro if the air resistance in the form of frictional drag were absent.
However, such a drag force exists and it exerts a mild opposing force on the pendulum. Thus, although it appears to be moving to and fro endlessly, actually a pendulum comes to rest after a sufficiently long time.
This is known as the damping of the oscillation. The case of a ball dropped from a height on to the floor is different. It bounces from the floor, during which the ball loses a fraction of its energy by deforming itself and the floor at the point where it hits.
The ideal case, where no energy is lost during bouncing, is unachievable in practice. On its rising track it is acted on by two forces, one, the downward force of gravity and two, the air resistance. The air resistance reduced the energy further. Hence, the ball does not rise to as much height as it was originally dropped from.
The ratio of the height of a bounce to that of the previous bounce is known as the coefficient of restitution and is a property of the combination of the materials of the ball and the floor. This process repeats in the subsequent bounces and the height to which it can rise keeps reducing. This leads eventually to stop the ball from further rising and bouncing.
Dr. H.K. Sahu
Scientific Officer
IGCAR, Kalpakkam, Tamil Nadu
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