Momentum

Momentum is mass times velocity:

$$\vec{p} = m\vec{v}$$

It points the same way as the velocity, and its unit is kg·m/s. On a line, a momentum the negative way is negative.

A light object can match a heavy one by moving faster: $1\ \mathrm{kg}$ at $8\ \mathrm{m/s}$ and $4\ \mathrm{kg}$ at $2\ \mathrm{m/s}$ both have $8\ \mathrm{kg\cdot m/s}$.

A light, fast cart and a heavy, slow cart can carry the same momentum.