Physics — Centre of Mass & Collisions

\mathbf{r}_{cm} = \frac{\sum_i m_i \mathbf{r}_i}{\sum_i m_i}

\sum p_x = \text{const}, \quad \sum p_y = \text{const}

e = \frac{v_2' - v_1'}{v_1 - v_2}

KE_i = KE_f \;\;\text{(elastic)}