Stability and States of Equilibrium

Stability refers to the ability of a body to restore to its original static equilibrium, after it has been slightly displaced.

• A body in equilibrium experiences neither linear acceleration nor turning effect and, unless disturbed by an outside force, will continue in that condition indefinitely. (Newton’s First Law)

States of Equilibrium

Stable Equilibrium

A equilibrium is said to be stable if small, externally induced displacements from that state produce forces or torques that tend to oppose the displacement and return the body to its original equilibrium.

Example: A hanging pendulum or a brick lying on a level plane.

Unstable Equilibrium

An equilibrium is said to be unstable if the least departures produce forces or torques that tend to increase the displacement.

Example: A rule standing on its end or a ball standing on top of an inverted bowl.

Neutral Equilibrium

An equilibrium is said to be neutral if small, externally induced displacements from that state does not produce any unbalanced forces or torques that tend to oppose or aggravate the displacement and the body remains in its new equilibrium.

Example: A ball or a pencil lying on a level surface.

Summary of Different States of Equilibrium

	Stable	Unstable	Neutral
Base Area	Large	Small	1 line of contact or point(s) of contact with surface
Height of C.G.	Low	High	–
Slight displacement from equilibrium position	Return to original position	Topple over	Stay in new position

Criteria of Stable Bodies

Stable bodies tend to have:

• Low centre of gravity
• Wide base area

Hence, the stablity of an object can be improved by:

• Lowering its centre of gravity
• Increasing the base area