Inertia is defined as?

• A. speed
• B. friction
• C. momentum
• D. resistance to change in motion

Answer: (D)

Inertia is the resistance to a change in motion. It’s a property of matter that ties to mass: the more mass something has, the harder it is to start moving, to stop, or to change its direction or speed. In driving terms, this shows up as the feeling you experience when a car accelerates—you’re pressed back into your seat because your body resists the faster motion. When the car brakes, your body tends to keep moving forward, which is why seat belts and other restraints are essential—they apply the force needed to change your motion.

This idea isn’t the same as speed, which is just how fast you’re going. It isn’t a force like friction, which opposes motion. And it isn’t momentum, which is mass times velocity and describes the motion itself, not the resistance to changing that motion.