# Conservation of momentum: One moving car hitting another stationary one. Does the target always move forward?

Consider an isolated system with two cars of any mass, a ground with friction, and Earth. Both cars are free to move on the ground.

One car (Car #1) is moving towards the other stationary car (Car #2) at a constant velocity. Based on the conservation of momentum, when they collide, the momentum of Car #1 is transferred to Car #2, which means Car #2 carries non-zero velocity. Then here is my question:

Does it mean Car #2 moves forward in any case? That does not add up, because if the force by #2 on #1 is not enough to overcome the static friction by the ground on #1, Car #2 will "stay" on the ground, right? Or is it possible that the momentum transfer occurs as the forward deformation of the internal molecular structure (with non-zero velocity) of Car #2, while Car #2 is still fixed on the ground?

Thank you in advance.

• Yes. Your statement is correct and clear enough.
– ytlu
Mar 26, 2021 at 3:19
• That convinces me more. Thank you! Then it means the same thing happens when you punch a fixed wall. The wall does not physically move or slide, but it is internally deformed as the momentum transfers into it Mar 26, 2021 at 3:58