it does throughout. In a frame of reference in which the car on the right were stationary before the collision, however, this collision would look just like the previous example: the left hand car would appear stationary after the collision. For the following examples, the spring has been replaced by a piece of putty. The deformation of a spring is reversible, which gives an elastic collision. The deformation of putty is not at all reversible: all of the energy that goes into deforming it is lost (as heat in the putty). The collision is elastic, but conservation of momentum still applies so, as in the first example above, we have: mv + 0  =  mv 1 + mv 2 and v  =  v 1 + v 2 (5). Here, of course, we cannot apply conservation of mechanical energy. Instead, we know that the two cars stick together after the collision, so v 1 =  v 2 (6). Combining (5) and (6) gives v 1 =  v 2 =  v/2. Because the two masses are equal, the centre of mass is halfway between them. Before the collision, one car had velocity v and the other zero, so the centre of mass of the system was also v/2 before the collision. The total momentum is the total mass times the velocity of the centre of mass, so the total momentum, before and after, is (2m)(v/2) = mv. In the next case we'll show, the intitial velocities are equal in magnitude and opposite