[jncfhnb]

I still disagree. Moving is not sufficient. If both are moving in the same direction and one hits you in the back, you would not say they collided with each other.

we define collisions by whether the velocity facing side of the object hits the object.

[SideburnsOfDoom]

> If both are moving in the same direction and one hits you in the back, you would not say they collided with each other

you are talking about a "Rear-end collision", correct? https://en.wikipedia.org/wiki/Rear-end_collision

[jncfhnb]

Yes. You would not say the lead car collided with the other in a rear end collision unless it was reversing.

> Typical scenarios for rear-ends are a sudden deceleration by the first car (for example, to avoid someone crossing the street) so that the driver behind it does not have time to brake and collides with it.