When one kid runs into another and they bump heads, why is the slowest mover in most pain?
In my experience, if one kid runs really fast and accidentally bumps into a second kid who's standing still, and they bump heads, the kid who was 'standing still' (relative to earth) always seems to be impacted heavily (more 'damage received'), while the 'runner' doesn't suffer much physical damage at all. This suggests that both 'objects' in a collision scenario seem to 'experience' the same event in different ways while relativity tells us the point of view of both objects is equivalent - they are both equally valid ways of analysing the event.
So: shouldn't both kids feel the same impulse and be about 'equally damaged'? Since movement is relative, we might just as well view the situation from the point of view of the running kid (rather than the point of view of the kid standing still), and we then see the kid who's standing still approach us equally fast and bump heads in the exact same way. Looking at it this way, the kid standing still hit the runner in the head, so the runner should receive more physical damage. 174
Since both points of view are equally valid, the only solution is that both should feel the exact same impulse (= equal 'damage').
Of course the details of any specific collision are a complex issue, but there is a trend here, a large discrepancy in 'favour' of the runner, that is not explained by merely calling upon differences in built, strength, ... Equivalent kids experiencing such a scenario where one kid just stands there while the other smashes into him at top speed result in the runner pretty much running through the stationary kid and hardly noticing, while the stationary kid is knocked out and receives much more physical damage. It's never the other way around.
So why do we notice a difference in practice?
(I guess we see the same effect in, for example, a soccer player performing a header: If another player shoots really hard and the ball hits another player's head, it hurts badly if that player is standing passively - even if the player is massive and strong - and a lot less if the other player actively moves towards the ball - even if the player is a relatively 'weak' person, and even though the speed with which both objects are moving towards each other is even greater when one moves actively towards an approaching object.) 174