What conditions must be satisfied for a rigid body to be in static equilibrium?

• A. Net force and net torque about any axis are zero.
• B. Net force zero only
• C. Net torque zero about any axis is zero
• D. Net energy zero

Answer: (D)

In static equilibrium for a rigid body, there is no motion at all—neither translation nor rotation. This happens only if two things hold: the sum of all external forces acting on the body is zero, so there’s no linear acceleration, and the sum of all external torques about any axis through the body is zero, so there’s no angular acceleration. In other words, the forces balance and the torques balance.

Energy isn’t what fixes the body in place. The total mechanical energy can be nonzero and still be in equilibrium—as long as the forces and torques balance. For example, a object resting on a support can have gravitational energy but remain stationary because the upward support balances the weight and any resulting torques cancel.