Which statement best defines inertia?

• A. A body in motion tends to stay in motion unless acted upon by a force
• B. The energy of motion
• C. The resistance of a body to change its motion
• D. A body at rest will stay at rest and a body in motion will stay in motion unless acted upon by an external force

Answer: (D)

Inertia is the tendency of matter to resist changes in its state of motion or rest. In plain terms, objects keep doing what they’re doing unless something else (a force) makes them change.

The best statement captures both aspects: a body at rest stays at rest and a body in motion stays in motion unless acted upon by an external force. That is Newton’s first law in its complete sense, showing inertia as the resistance to changing how an object is moving (or not moving).

The other options are less accurate for defining inertia. Saying a body in motion tends to stay in motion focuses only on moving objects and ignores objects at rest. Describing inertia as simply “the resistance of a body to change its motion” is close, but it doesn’t explicitly state that it also applies to objects at rest and the role of external forces in causing the change. Calling inertia the energy of motion confuses it with kinetic energy.

A practical example: a parked car remains stationary until someone pushes or applies the brakes; a moving car would keep moving in a straight line unless friction or a brake force acts on it. This illustrates inertia in action.