Lets suppose a vehicle (A) stopped at a red light is rear ended by another vehicle(B). Both of the cars slid with locked wheels until frictional forces bring them to a stop. When trying to find the velocity of the second car just before impact and assuming that linear momentum is conserved in the collision I would use : Vb=(MaVa+MbVb)/Mb Now my question is, what possible ways could this assumption be invalid? I know that the conservation of lin. momentum during the impact depends on the fact that the only significant force is the force of contact between the cars. Am I correct in saying that is in turn ignores the force of friction exerted by the road on the cars during the impact and this would cause error in calculation?