Two particles of equal mass have velocities $\vec{v}_1 = 4\hat{i}\ \text{ms}^{-1}$ and $\vec{v}_2 = 4\hat{j}\ \text{ms}^{-1}$. First particle has an acceleration $\vec{a}_1 = (5\hat{i} + 5\hat{j})\ \text{ms}^{-2}$ while the acceleration of the other particle is zero. The centre of mass of the two particles moves in a path of