Equilibrium is satisfied when which conditions hold true?

• A. The net force on a system is zero and the net moment is zero.
• B. The system is in motion.
• C. The force equals energy.
• D. The fluid pressure increases.

Answer: (A)

Equilibrium means there is no tendency for the body to change its motion, either by translating or rotating. For a rigid body, this requires two things to be true at once: the sum of all external forces must be zero, and the sum of all external moments (torques) about any point must also be zero. When the net force is zero, there’s no translational acceleration; when the net moment is zero, there’s no angular acceleration, so the body doesn’t start or stop rotating.

If only the net force is zero but there’s a nonzero net moment, the body would rotate even though it isn’t accelerating linearly. If the net moment is zero but there’s a nonzero net force, the body would translate but rotate as well. So both conditions must hold for true equilibrium.

Other statements don’t describe equilibrium: simply being in motion isn’t a guarantee of equilibrium, and “force equals energy” isn’t a relevant criterion for balanced forces and moments; changing fluid pressure affects the balance of forces and moments, not the equilibrium condition by itself.