Conditions for Static Equilibrium
If a rigid body is in “static equilibrium,” it is at rest -- no translational acceleration and no rotational acceleration. BOTH of the following must be true for any body in static equilibrium:
The vector sum of external forces must be zero:
The sum of torques due to all external forces about any axis must also be zero:
This slide defines what w mean by “static equilibrium.” Since torque is defined in terms of force, each of these conditions of equilibrium represent a mathematical statement about the applied forces that must be true if a rigid body is in static equilibrium. These conditions of static equilibrium are often used to predict the static forces -- tensions and other stresses -- that exist in structures that are in static equilibrium.