Momentum

$$\begin{align*} p &= mv \\ \text{momentum} &= \text{mass} \cdot \text{velocity} \end{align*}$$

Unit of $p$ = $\mathrm{kg\,ms^{-1}}$

Impulse

$$\begin{align*} \Delta p &= Ft \\ &= m(v-u) \\ \text{impulse} &= \text{resultant force} \cdot \text{change in time} \\ &= \text{mass} \cdot (\text{final velocity} - \text{initial velocity}) \end{align*}$$

Unit of $\Delta p$ = $\mathrm{N\,s}$
Unit of $\Delta p$ = $\mathrm{kg\,ms^{-1}}$

Principle of Conservation of Momentum

In a system of colliding bodies, the total momentum remains constant if there is no resultant external force

Derived from Newton's 3rd Law:
When two objects collide, an opposite pair of forces is produced.
The areas under the force-time graphs of the two objects (which is equal to momentum) cancel each other out.