Which of Newton's laws is commonly called the law of inertia?

• A. First Law
• B. Third Law
• C. Gravity
• D. Second Law

Answer: (A)

Inertia is the tendency of objects to resist changes in their motion, and Newton's first law states that an object at rest stays at rest and an object in motion stays in motion unless a net external force acts on it. This is why a ball sitting on the ground remains still until someone kicks it, and a ball rolling on a smooth surface keeps moving in a straight line at a constant speed until friction or another force slows or redirects it. The concept of inertia ties directly to mass—the more mass something has, the greater its resistance to changing its motion.

In sports terms, you must apply a force to start moving a stationary ball, to stop a moving ball, or to change its direction. That need for a net external force is what makes this law the one commonly known as the law of inertia. The other Newton laws deal with how force, mass, and acceleration relate, or with action–reaction pairs, or with gravity as a force—not with inertia itself.