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.

  • $\begingroup$ Yes. Your statement is correct and clear enough. $\endgroup$
    – ytlu
    Mar 26, 2021 at 3:19
  • 1
    $\begingroup$ 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 $\endgroup$
    – TBS500
    Mar 26, 2021 at 3:58

1 Answer 1


If Car 2 is immovable then the momentum that is lost by Car 1 in the collision is actually “transferred” to the Earth. Of course, the large mass of the Earth means this makes a negligible difference to the Earth’s motion through space.

  • $\begingroup$ When you said "immovable", it sounds like the car is "glued" on the ground and not free to move. Is that right? $\endgroup$
    – TBS500
    Mar 26, 2021 at 21:07
  • $\begingroup$ @TBS500 I just mean that it does not move - either because it is fixed to the ground or because it is so heavy that static friction is sufficient to prevent it from moving. In fact, even if Car 2 does move, any non-zero friction will transfer some momentum to the Earth because of Newton's Third Law. $\endgroup$
    – gandalf61
    Mar 27, 2021 at 6:43

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