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
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.
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.
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
|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