What statement is true about a system of forces that is in equilibrium?

In a system of forces that is in equilibrium, the essential condition is that the sum of all forces acting on the system equals zero. This means that all the forces are balanced, and as a result, the total upward forces are equal to the total downward forces. When this balance is achieved, the object remains in a state of rest or moves with a constant velocity, indicating that there is no net force acting on it.

This principle is fundamental in various applications, such as in structural engineering and mechanics, where maintaining balance is crucial for stability. If the forces were not balanced in such a scenario, the object would either accelerate in the direction of the net force or begin to rotate, depending on how the forces are applied. Therefore, the statement about the total upward forces being equal to the total downward forces accurately describes a key aspect of a system's equilibrium.

The other options all suggest conditions that would contradict equilibrium, such as rotation or vibration, which would signify that there are unbalanced forces acting on the system.