6.5. Newton’s Famous Three laws

Newton’s momentum and mass are at the heart of his three laws of motion. Let’s go through them in words, and then one of them in more detail algebraically.

Newton’s 1st law of motion says that anything that’s moving at a constant speed (which could be zero) will continue in that way unless a force acts on it. That’s a statement about inertia—resistance to acceleration. (Newton inherited this idea from Galileo, Descartes, Isaac Beeckman, and Marin Mersenne, but gave it a quantitative meaning.)

Newton’s 2nd law of motion says that momentum is changed when a force acts on an object for a duration of time. Or, you might have learned it as a defining statement about “force” namely:

Force is equal to the rate of change of momentum.

Newton’s 3rd law of motion is subtle, and solely due to Newton’s ingenuity. It says that if you push on something—anything and with any amount of force—that object—will push back with exactly the same force. We’ll think harder about the 3rd law when we talk about collisions.