Caption: Mechanical stability illustrated.

The left-hand system is in stable equilibrium.

Small perturbations will NOT permanently change the system.

Gravity will pull the ball back to the bottom of the valley and friction and drag will damp out motion.

The right-hand system is in unstable equilibrium.

Any small perturbation will start the ball rolling away and it will NOT come back without some new process acting.

Credit/Permission: © David Jeffery, 2003 / Own work.
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