How is Inertia described within laws of motion?

• A. A body at rest will remain at rest and a body in motion will remain in motion in a straight line until acted upon by an external force.
• B. The tendency of objects to maintain their state of motion is called inertia.
• C. The rate of change of velocity is proportional to force.
• D. Momentum is conserved for all systems.

Answer: (A)

Inertia is the property of matter that resists changes to a body’s motion. When this idea is placed in the laws of motion, it’s captured by Newton’s first law: a body at rest stays at rest, and a body in motion keeps moving in a straight line unless an external force acts on it. This phrasing ties inertia directly to observable behavior—without outside influence, an object won’t alter its motion. The other statements either define inertia in a general sense or describe other ideas (how velocity changes with force, or momentum conservation), which don’t describe inertia itself within the laws as clearly. So the best description is the one that shows inertia manifesting as motion persisting in a straight line or staying still unless something from outside changes it.