system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Which would cause more damage: 2 equal sized cars ramming head on, both going 60mph OR 2 equal sized cars, 1 parked, the other going 120mph?
gpt
Roughly the same. One major postulate of physics is that all reference frames are equally valid.

So there is no difference between two cars moving towards each other each at 60 and one car moving towards the other at 120.

I say roughly, because in real life there's retarding forces, material properties, and several other factors which will impact what happens. But in theory, the two are the same. In fact, there's no distinguishing from the two situations. 

edit: an additional answer from Quora

>We assume cars of equal mass, and the collision is inelastic. Then, yes, it's equivalent. You can think of this as a result of (ordinary) relativity. In the 120+0 case, consider moving in a reference frame that is going 60mph w.r.t. the earth. Then it is clear; you would see nothing different than if you were stationary and the cars do 60+ -60.

>But you may ask: Does not the 120+0 case have twice the initial kinetic energy than the two 60's case? (120^2 is 4 times 60^2+60^2) The answer to this is that the combined mass of the two cars in the 120+0 case, by conservation of momentum,  continues to travel at 60mph. Thus not all, but only half, the initial kinetic energy is used up in the collision.