Equilibrium, a state of balance between opposing forces. In mechanics, an object at rest is in a state of equilibrium. Such an object will move only if the balance of forces acting upon it is upset. If this balance is relatively difficult to upset, the object is in stable equilibrium. A good example is a cone resting on its base on a horizontal surface. An object in unstable equilibrium is easily overturned, as is a cone placed on its tip. An object in stable or unstable equilibrium is overturned by tilting it until its base is no longer under its center of gravity.

An unstable object has a high center of gravity and a small base. The center of gravity is easily shifted by slight force, causing the object to fall over. A stable object has a broad base and its center of gravity is low. It can be overturned only by tilting it so far that a vertical line drawn through the center of gravity clears the base. In an object in neutral equilibrium—such as a wheel, a ball, or a cone resting on its side—the center of gravity coincides with the center of rotation and cannot be shifted by tilting.

Biological equilibrium is the coordination of muscular forces that maintain an animal in its normal posture. It is controlled by the sense of balance. Chemical equilibrium is the state in which one chemical reaction balances another. Such a state is indicated in a chemical formula by two horizontal arrows, one over the other, pointing in opposite directions. Thermodynamic equilibrium is a condition in which temperature and pressure do not fluctuate.