What is inertia?

• A. The measure of how much matter is in an object.
• B. The force that keeps an object moving at constant speed.
• C. All objects resist a change in their motion.
• D. The energy of an object due to motion.

Answer: (C)

Inertia is the tendency of any object to resist changes in its motion. The more mass an object has, the greater its inertia, so it’s harder to start moving or to bring something that’s moving to a stop. In a frictionless world, an object in motion would keep moving at the same speed and direction unless a force acts on it. In the real world, forces like friction or collisions cause changes in motion, and inertia is what makes those changes feel noticeable.

That’s why heavier objects resist starting to move or stopping more than lighter ones. It’s also why you lunge forward when a car stops quickly—the body wants to keep moving, and a seatbelt or airbag provides the force needed to change that motion. This concept is different from mass itself, and it’s not the same as a force or the energy an object has while moving.