I'm working on a momentum lab right now and I'm having a hard time finding the sources of error. What we did was take two cars. One was moving and one wasn't, and we had them collide and stick together. Then we had to figure out the change in momentum for each car. I thought my teacher mentioned something about having to assume that the masses stay constant thoughout the experiment. I don't know if this seems like a valid source of error or not. Also, does it matter if the moving car wasn't travelling at a constant velocity? Since there is no acceleration in the formula p=mv, I want to say no, but I'm not sure. (We measured the velocity at different time intervals and at the end used the average momentum).