What is inertia?

• A. resistance to change in motion (a body in motion remains in motion, a body at rest remains at rest)
• B. a motion in a straight line
• C. a type of friction
• D. energy of motion

Answer: (A)

Inertia is the tendency of objects to resist changes to their motion. This means a moving body keeps going at the same speed and direction unless a force acts to slow it down or alter it, and a stationary body stays put until a force starts it moving. The more mass something has, the greater its inertia, so more force is needed to change its motion. In driving, inertia explains why you lurch forward when a car stops suddenly and why seat belts and brakes are needed to counteract that tendency. The other ideas aren’t correct here: inertia isn’t a motion in a straight line, it isn’t a type of friction, and it isn’t energy of motion (that would be kinetic energy).