Inelastic collision problem A car of mass  $1400$ $\mathrm{kg}$    moving  south  at  $11$  $\mathrm{ms}^{−1}$  collides  into  another  car  of  mass  $1800$ $\mathrm{kg}$  moving  east  at  $30$ $\mathrm{ms}^{−1}$.  The  cars  are  stuck  together  after  the  collision.  Determine  the velocity  of  the  cars  immediately  after  the  collision.
I know the problem can be easily solved by conservation of momentum before and after collision. But I get confused in the direction in which the system will move after collision. I have solved many problem but I make a mistake every time. I have taken this very basic problem to understand the concept properly. Please throw some insight so that I can get a clear picture of after collision every time without making a mistake.
 A: The way to approach this problem is to use the vector momenta and conserve them. If we orient our coordinate axis such that our $x$-coordinate is pointing North and our $y$-coordinate is pointing East, then the momentum of the first car is:
$$\vec{p}_{1}=m_{1}\vec{v}_{1} = 1400\begin{pmatrix}-11 \\ 0\end{pmatrix}\:\text{kg m s}^{-1}$$
Similarly, the momentum of the second car is:
$$\vec{p}_{2} = m_{2}\vec{v}_{2} = 1800\begin{pmatrix}0 \\ 30\end{pmatrix}\:\text{kg m s}^{-1}$$
Conservation of momentum means that:
$$\vec{p}_{\text{final}} = \vec{p}_{1} + \vec{p}_{2} = \begin{pmatrix}-15400 \\ 54000\end{pmatrix}\:\text{kg m s}^{-1}$$
But $\vec{p}_{\text{final}} = (m_{1} + m_{2})\vec{v}_{\text{final}}$ and so we can find the velocity of the cars after the collision:
$$\vec{v}_{\text{final}} = \frac{1}{1400+1800}\begin{pmatrix}-15400 \\ 54000\end{pmatrix}\:\text{m s}^{-1} = \frac{1}{16}\begin{pmatrix}-77 \\ 270\end{pmatrix}\:\text{m s}^{-1} \approx \begin{pmatrix}-4.8 \\ 16.9\end{pmatrix}\:\text{m s}^{-1}$$
So the joint cars are travelling east at approximately $16.9\:\text{m s}^{-1}$ and south at approximately $4.8\:\text{m s}^{-1}$.
