In an inelastic collision momentum is conserved, but kinetic energy isn't??? Here is a simple example about my issue with this. 1g ball (ball A) moving west at 10m/s hits another 1g ball (ball B) moving east at 10m/s. After the collision, ball A moves east at 3m/s, and ball B moves west at 3m/s. My understanding is that momentum is conserved because the sum of momentum before the collision will equal the sum of momentum after the collision. If that is correct, then why doesn't it work that way with kinetic energy? Regarding the example above, isn't the net kinetic energy of the system 0m/s before the collision and 0m/s after the collision? In other words, my stubborn intuition tells me that there should be no difference between a vector quantity and a scalar quantity in a two dimentional collision.