What does it mean for a mechanical system to be in equilibrium?

When a mechanical system is in equilibrium, it signifies that all the forces acting upon the system are balanced. This means that the total vector sum of the forces is zero, resulting in no acceleration. The system either remains at rest or continues to move at a constant velocity, depending on its initial state. The state of equilibrium indicates stability, where no net external force is causing a change in motion.

In contrast, the other options describe conditions that do not reflect equilibrium. If there were a net movement in one direction, the system would be accelerating, indicating that forces are not balanced. A system powered by an external force typically implies that there is a net force acting on it, again suggesting that it is not in equilibrium. Lastly, the presence of multiple moving parts does not inherently relate to equilibrium; a system could be in equilibrium even with many components, as long as the forces on those parts are balanced. Understanding this balance is crucial for analyzing mechanical systems in various applications.