The principle that the resultant remains unchanged when another system in equilibrium is added or removed is described by which law?

• A. Parallelogram Law
• B. Equilibrium Law
• C. Action and Reaction
• D. Superposition Law

Answer: (D)

The principle being tested is the Superposition Law. In statics, the effects of multiple force systems simply add vectorially. If you bring in another system that is in equilibrium, its resultant is zero, so adding or removing that system does not change the overall resultant on the body. That invariance under adding a zero-resultant system is exactly what the Superposition Law describes.

The parallelogram Law is a method for constructing the resultant of two forces, which is related but doesn’t capture the idea that adding a separate equilibrium system leaves the resultant unchanged. Action and Reaction refers to Newton’s third law between two bodies, not to the effect of introducing another equilibrium system. Equilibrium Law isn’t a standard, specific term for this concept.