ABSTRACT
The train can balance at the high point of a roller coaster but a slight disturbance causes it to move rapidly away from its equilibrium position. The train is said to be in unstable equilibrium. Compare this with the swing boat. When this is disturbed, the resulting net forces will tend to restore it to its equilibrium position. The swing is in stable equilibrium.
You would like a garage door to be in equilibrium in any position so that a breath of wind does not bring it crashing on to your car. It is safest when the forces acting are always balanced so that it is in neutral equilibrium.
