For school I have to explain how whiplash can occur if a driver is at a read light and he seat doesnt support his head, and he gets hit from behind. Now I was thinking that I needed to find the velocity of the car after it had been hit from behind, so that I could find the corce acting on the drivers head. Upon investigation I found that collisions have to do with momentum, But i was unable to find out the key thing i was looking for, which is : If car one is stopped at a red light, and car 2 is moving at a known velocity (and you know both cars masses) how do u figure out the velocity of each car after a rear collision? I've been looking at this site so far and it has confused me: http://www.physicsclassroom.com/mmedia/momentum/momentumTOC.html [Broken] On that site there are examples where the ars bounce backwards after impact, is this wha owuld happen on a normal road? I would think that either car 2 would stop and car 1 would get car 2's velocity, or that both of there velocitys would be positive after collision. Also would the force of friction on the road cause momentum to be lost after the collision? or can I assume it's negligable for this car crash senario?