State Newton's first law in the context of a stationary object on a frictionless surface.

• A. An object at rest remains at rest unless acted on by a net external force.
• B. An object at rest will stay at rest even if a net external force acts.
• C. A stationary object will move if any external force is applied.
• D. The net external force on a stationary object is always zero.

Answer: (A)

Inertia is the idea that objects resist changes in their motion. On a frictionless surface there’s no opposing force from friction, so the only thing that can change whether the object stays put or starts moving is an unbalanced (net) external force. If nothing pushes or pulls, there’s no net external force and the object remains at rest. If a net external force acts, that unbalance causes the object to accelerate and move. So the statement that an object at rest remains at rest unless acted on by a net external force captures this idea precisely and describes how motion changes only when an unbalanced force is present.

B is not correct because it says a net external force won’t change the state of rest. C aligns with intuition in this frictionless setup—that any applied external force will cause motion—but the most exact phrasing for Newton’s first law emphasizes the need for an unbalanced force to change motion. D is incorrect because the net external force on a stationary object is not always zero; it becomes nonzero when a force is applied, and that’s what starts the motion.