I've been trying to get a good grip on the difference between conservation laws. Momentum is particularly tricky, I don't understand how quantities like $m\mathbf v$ can be conserved when other things like deformation and heat come into play. Googling it seems to suggest that the answer lies in energy being a scalar and momentum being a vector, but then this situation still confuses me:
Two cars with the same mass and different velocities, say $-5\mathbf i$ and $10\mathbf i$, collide and no external forces act.
All their kinetic energy goes into sound, heat, and deformation. So final velocity is reduced to zero for both of them.
What happened to the momentum? Their vectors were unequal so it can't be zero, but the final velocities are zero.
Thanks for any help.