Equilibrium

• A body which is not moving is in a state of equilibrium.
• A body is in a state of stable equilibrium when any small movement increases its potential energy so that when released it tends to resume its original position.
• A body is in a state of unstable equilibrium if, following any small movement, it tends to move away from its original position.
• The sum of reaction forces on a body is always equal to the sum of action forces if the body is in a state of equilibrium, regardless of the positions and magnitudes of the action forces.
• If a body is in an equilibrium state, all the moments acting on the body about any point should be zero.