Description: An object is in a state of equilibrium when the sum of forces is equal to zero. 
Key Terms: Equilibrium, scale, gravity, tension, support force, balance, normal force

In the figure, we see the forces involved for Burl and Hewitt on their signpainting scaffold. The sum of the upward tensions is equal to the sum of their weights plus the weight of the scaffold. Note how the magnitudes of the two upward vectors equal the magnitude of the three downward vectors. Net force on the scaffold is zero, so we say it is in mechanical equilibrium.

Rest is only one form of equilibrium. An object moving at constant speed in a straight-line path is also in equilibrium. Equilibrium is a state of no change. A bowling ball rolling at constant speed in a straight line is in equilibrium—until it hits the pins.