Newton's first law states that an object at rest stays at rest unless acted on by what?

• A. Net external force
• B. Friction
• C. Gravity
• D. Inertia

Answer: (A)

An object at rest remains at rest only when there is no unbalanced force acting on it. The phrase “net external force” means the total of all forces acting from outside the object. If that net force is zero, the object’s motion doesn’t change—so it stays at rest. If the net force is not zero, the object accelerates in the direction of that net force, as described by F = ma. Individual forces like gravity or friction matter only insofar as they contribute to the overall (net) force; they don’t by themselves determine the outcome unless they produce a nonzero net result. Inertia is the tendency to resist changes in motion, but the actual change in motion happens because of a nonzero net external force.