What is inertia?

• A. The momentum of a moving car
• B. The energy of motion
• C. A force that accelerates objects
• D. A resistance to change a state of motion

Answer: (D)

Inertia is the resistance to a change in motion. It means an object tends to keep doing what it’s already doing: a stationary object stays at rest, and a moving object keeps moving in the same direction at the same speed unless something else (a force) acts on it.

The amount of inertia depends on mass—the more mass an object has, the more it resists changes in motion. That’s why a heavy cart is harder to push to start or stop than a light one.

Think of everyday situations to see inertia in action. When a car suddenly stops, your body tends to keep moving forward because of inertia unless the seatbelt provides the force to change your motion. Friction and air resistance are other external forces that can eventually slow or stop moving objects.

This concept is different from momentum (which is mass times velocity) and from kinetic energy (the energy of motion). It’s also not a force that accelerates objects; it’s the property that makes forces necessary to start, stop, or change motion in the first place.