## 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.

```{aside}

Your immediate reaction might naturally be to wonder how anything moves at all, since there appears to be a balance from this rule. What matters for an object to feel an imbalanced force on it. That it exerts a force on something else doesn't affect its own motion. So, if a donkey pulls on a wagon, the wagon pulls back...but the force on the wagon is itself imbalanced and so it moves.

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**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.