A rigid body is in mechanical equilibrium when the sum of the forces, and torque, experienced by the system is zero.
Equilibria can be stable or unstable:
- Stable equilibrium: The response to a small perturbation in forces and torque that tends to restore the equilibrium.
- Unstable equilibrium: The response to a small perturbation in forces and torque that tends to move the system even farther away from its original point of equilibrium (actually, the system often returns to a new stable equilibrium after the disturbance!).
This animation permits the illustration of the forces involved. The position of the center of gravity (or center of mass) of the system, in relation to its support (balancing point) is the important element here.