[jjulius]

>Idk. If you say "two objects collided" that to me implies a mutual head on collision.

The commonly used phrase for such a scenario is "head-on collision", while "collision" just means objects coming together with a solid impact - could be one object moving, could be both.

>You wouldn't say anything object at rest collided with something else.

No, but you'd say that the car collided with the tree rather than the tree and the car collided. In this specific instance, both cars were moving, and they collided with each other.

[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.