In physics, equilibrium is achieved when which condition is met?

In physics, equilibrium is achieved when which condition is met?

• A. A device that moves a liquid from low pressure to high pressure.
• B. The net force on a system is zero.
• C. The action of forces and moments on a system can be summed to zero.
• D. The natural oscillation frequency of a structure.

Answer: (C)

Equilibrium means there is no tendency for the system to move or rotate, so all external effects cancel out. For a rigid body this requires both the translational and rotational influences to balance: the sum of all forces must be zero and the sum of all moments (torques) about any point must be zero. This is why the statement that the action of forces and moments on a system can be summed to zero best describes equilibrium. If only the net force were zero but a remaining net moment existed, the body would rotate, so that condition wouldn't truly be equilibrium. The other options describe a device moving fluid against pressure, or focus on a natural oscillation frequency, neither of which captures the full balance needed for equilibrium.

Equilibrium means there is no tendency for the system to move or rotate, so all external effects cancel out. For a rigid body this requires both the translational and rotational influences to balance: the sum of all forces must be zero and the sum of all moments (torques) about any point must be zero. This is why the statement that the action of forces and moments on a system can be summed to zero best describes equilibrium. If only the net force were zero but a remaining net moment existed, the body would rotate, so that condition wouldn't truly be equilibrium. The other options describe a device moving fluid against pressure, or focus on a natural oscillation frequency, neither of which captures the full balance needed for equilibrium.