Lets say that I have a cart on a track with silly putty on the end. This first cart is moving with an initial "x" velocity. There is another cart about a foot ahead of it with sillyputty on its bumper also. (Both carts have the same mass) This cart has an initial velocity of zero. Of course when the two cars collide, they will stick and move together. I am just trying to understand this through Newton's third law. When the first moving cart hits the second cart, the first cart is applying a force to accelerate the nonmoving car, and the nonmoving cart is applying an equal and opposite reaction force on the moving car, slowing the moving cart down. Why does the reaction force the initially nonmoving cart is applying on the initially moving cart not completely slow down the initially moving cart? I know that in this case in an elastic collision, the first cart would completely stop and the second cart would take all of the momentum of the first cart. So what is it about the inelasticity of the collision that overrides the fact that the first cart is not able to transfer all of its momentum to the second cart?