Out of the following which one is an example of dynamic equilibrium ?
(1) Moon revolving around the earth
(2) A pebble fixed at the end of a string whirling in a circular path
(3) An aeroplane moving at a constant height
(4) A beam balance balanced in a horizontal position.
(a) (3) and (4)
(b) (1), (2) and (3)
(c) (1) and (2)
(d) (1) and (3)

(b) (1), (2) and (3)

Reason — Dynamic equilibrium refers to a state where two or more opposing processes occur at the same rate, resulting in a stable overall condition.

1. Moon revolving around the earth is an example of dynamic equilibrium as the gravitational force pulling the moon towards the Earth is balanced by the centrifugal force of the moon’s motion, resulting in a stable orbit.
2. A pebble fixed at the end of a string whirling in a circular path is also an example of dynamic equilibrium as the tension in the string pulling the pebble towards the center is balanced by the centrifugal force of the pebble’s motion, keeping it in a stable circular path.
3. An aeroplane moving at a constant height is also an example of dynamic equilibrium. It achieves dynamic equilibrium when upward lift on it balances its weight downwards.
4. A beam balance balanced in a horizontal position is not an example of dynamic equilibrium because the balance may remain still, it’s a static equilibrium where opposing forces (gravity pulling down on each side of the balance) are already balanced without any ongoing processes happening.