Newton's first law of motion states which of the following?

• A. An object at rest stays at rest, and an object in motion stays in motion unless acted upon by a net external force.
• B. Force equals mass times acceleration.
• C. For every action there is an equal and opposite reaction.
• D. Work equals force times distance.

Answer: (A)

Inertia is what this is testing. Newton’s first law says an object resists changes to its state of motion: if there’s no net external force acting on it, an object at rest stays at rest and an object in motion keeps moving with the same speed and direction. The key idea is the net external force—when that sum is zero, velocity doesn’t change; when it’s not zero, the motion changes (acceleration in the direction of the net force).

Think of an object on a frictionless surface: if you give it a push, it keeps moving after the push stops only if there’s no other force slowing it down. In the real world, friction and other forces mean there’s usually a net external force that changes motion, which is why things slow down or change speed.

The other statements describe different ideas: one relates force to how quickly motion changes (second law), another talks about action and reaction forces (third law), and the last defines work as force times distance. These are important, but they describe how motion changes or how energy is transferred, not the persistence of motion or rest in the absence of a net external force.