Conservation of momentum is a fundamental principle that governs all interactions in the universe. It states that in any closed system the combined momentum at any one point in time must equal the momentum at any other point in time. Because our ball collider is approximately a closed system, the momentum will be approximately conserved throughout our experiment. However, because our system is not closed because of air resistance, momentum is not completely conserved in our system.

The law of conservation of momentum is: \[\vec p{_{\text{f}}} = \vec p{_{\text{i}}} + \vec F{_{\text{net}}} \Delta t\]

Deriving from this:

\[\begin{aligned} \vec p{_{\text{f}}} &= \vec p{_{\text{i}}} + \vec F{_{\text{net}}} \Delta t \\ \vec p{_{\text{1f}}} + \vec p{_{\text{2f}}} &= \vec p{_{\text{1i}}} + \vec p{_{\text{2i}}}+ \vec 0 \\ \vec p{_{\text{1f}}} - \vec p{_{\text{1i}}} &= \vec p{_{\text{2i}}} - \vec p{_{\text{2f}}} \\ \Delta \vec p {_{\text{1}}} &= - \Delta \vec p {_{\text{2}}}\end{aligned}\]