Question

Let us assume that a pencil is standing on its point or a stick in a vertically standing position. This will be an example of
A. stable equilibrium
B. unstable equilibrium
C. neutral equilibrium
D. None of the above

Answer 1

A system is considered to be in unstable equilibrium when it is made to move, it feels a resultant force or torque in the similar direction as the displacement from equilibrium. A system in unstable equilibrium accelerates away from its equilibrium point when displaced even slightly. This will help you in answering this question.

Complete answer:
A system can be considered to be in stable equilibrium when it is displaced from equilibrium, it will experience a resultant force or torque in a direction which is opposite to the direction of the displacement. A marble at the bottom of a bowl will be having a restoring force if it is displaced from its equilibrium position. This force makes it move backward towards the equilibrium point. Most systems will be in stable equilibrium commonly for small displacements. A system is considered to be in unstable equilibrium when it is made to move, it feels a resultant force or torque in the similar direction as the displacement from equilibrium. A system in unstable equilibrium accelerates away from its equilibrium point when displaced even slightly. A commonly seen example is a ball resting on top of a hill. When it is displaced from its equilibrium position, it will accelerate away from the crest. A system will be in neutral equilibrium when its equilibrium is not dependent on displacements from its initial position. A marble on a flat horizontal surface can be considered as an example. Here a pencil is standing vertically which is slightly disturbed from its position, then it will not come back to its initial position. Such kind of equilibrium is known as unstable equilibrium.

The correct answer will be option B.

Note:
Even the combinations of these situations are seen. A marble on a saddle will be stable for displacements towards the front or back of the saddle and unstable for displacements to the side. Vertically standing cylinder and funnel are other examples of unstable equilibrium.