Define inertia and relate it to Newton's first law.

• A. Inertia is the energy possessed by a moving object.
• B. Inertia is the force causing motion.
• C. Inertia is the resistance of any physical object to a change in its state of motion or rest.
• D. Inertia is the momentum of an object divided by its velocity.

Answer: (C)

Inertia is the resistance of any physical object to a change in its state of motion or rest. This means an object tends to keep doing what it’s already doing: a stationary object stays still, and a moving object keeps moving at the same speed and direction unless a net external force acts on it. Newton's first law expresses this idea: motion tends to continue unchanged unless something else changes it. The amount of inertia depends on mass—the more massive an object, the harder it is to speed up, slow down, or turn. That’s why a heavy object requires a larger push to change its motion. The other statements don’t define inertia: it’s not energy, not a force, and momentum divided by velocity equates to mass, which is related but not the definition of inertia itself.