Grade 11 asked what a force does while it acts. Grade 12 asks what it leaves behind, and the answer is momentum: $p = mv$, read aloud p equals m v. Three letters, three jobs — $p$ is the momentum, $m$ is the object's mass in kilograms, $v$ is its velocity in metres per second, and whichever one the question leaves blank is the one you solve for. The letter is p rather than m because m was already spoken for; it comes down to us from the older word impetus.

Momentum's unit is $\mathrm{kg \cdot m/s}$, and it has no nickname on purpose. The newton and the joule earned names because they turn up everywhere in disguise; momentum wears its recipe on the outside, so you can never forget what it is made of. Two things the unit will not tell you. First, momentum is a vector: declare a positive direction, and anything travelling the other way carries a NEGATIVE momentum — that minus sign is data, not decoration. Second, it is not kinetic energy. $E_k = \tfrac{1}{2}mv^2$ — $E_k$ in joules, same $m$ and same $v$ — squares the speed, so doubling a truck's speed DOUBLES its momentum and QUADRUPLES its energy. Same two ingredients, two different machines, and this chapter belongs to the first.