If two equal and opposite collinear forces are added to forces already in equilibrium, what is true?

When two equal and opposite collinear forces are added to a system that is already in equilibrium, those forces effectively cancel each other out. This happens because they are equal in magnitude and act in opposite directions along the same line. Since the original forces were in equilibrium, meaning the net force acting on the system was zero, the addition of these two collinear forces will not change that net force.

In terms of mechanical equilibrium, a system is said to be in equilibrium when the sum of all forces and the sum of all moments acting on the body are both zero. Because the new forces do not introduce any net effect—due to their equal and opposite nature—the sum of the forces remains at zero, and thus the original equilibrium state is preserved.

This concept is grounded in the principles of static equilibrium and the conditions that define it. Any addition of forces that do not result in a net force change will maintain the status of equilibrium, which is why the assertion regarding equilibrium being maintained is accurate in this context.