As someone who’s been on the wrong end of a butt as well I can tell you they are fearsome blows.
The only martial art you ever need to learn.
So? 
You don’t break your legs, because you allow your knees to bend, and may even drop and roll in an effort to extend the time of impact over as long a period as possible. Same impulse over a longer time of “collision” results in lower peak forces. Ever stepped off of a low step unexpectedly and taken a shock? You didn’t know the step was there, so your knee didn’t bend and spread the time of impact out.
Expecting a blow may make a difference psychologically, though…
There’s a debate about this?
You don’t need to be a billiard ball for the law of physics to apply to you. So you do get just as hard a hit as the other guy, but as you mention you are ready for it and hold your body/head a certain way, expect to feel some pain in a certain spot in advance, and so on. Just like heading a soccer ball or catching a baseball; knowing when and how some part of your body will make contact with these things allows you to make all the tiney adjustments that make taking the force relatively painless to you. You also know how much something can hurt when you’re not expecting it and/or take it in a soft spot.
- Someone throws a ball while you’re looking the other way that smacks you on the back of the hand or in the fingers and it could break them, yet you can catch the same ball sans pain at all if you see it coming and have a bit of coordination.
- You can jump into the air and have a soccer ball someone kicked from 25 yards away smack off you head and into a goal no problem, but you might collapse on the ground in disorientation and pain if I stood to your side and just threw the same ball at your ear or nose.
Now that sounds like a really difficult manuever. 
More an arguement, robby. I’ve heard arguements about which would do more damage. Two cars colliding head-on at 60mph each, or one traveling at 120mph running head-on into another that’s sitting still.
Assuming all four cars are exactly the same, of course. '75 Fleetwoods, for example. 
All four cars should sustain the same amount of damage. According to Newton, anyway.
Most people have no problem with the scenerio you have presented. I believe the argument arises when you try to explain that a car traveling at 60 mph that collides with an immovable object will experience the same impulse (and thus receive the same damage) as a car traveling at 60 mph that collides head-on with a car traveling (in the opposite direction) at 60 mph. (All car masses assumed identical.)
I know this has been discussed in GQ in the past.
Just tell them that each car, travelling at 60 mph, is an immovable object to the other car. That’ll clear things right up.
The football, aka soccer, heading analogy is quite good here. I am amazed at how hard of a hit I can take (and give) a soccer ball if I am prepared. Esp. if I hit “the sweet spot” on my forehead at the center of my hairline. OTOH, if I’m walking by a field and a ball whacks me when I’m not looking, it’s going to hurt. (I can take an expected soccer ball to any part of my head, but I can’t strike a ball hard outside of my forehead. Good soccer players can though.)
The “prepared” vs. “unprepared” seems to me to be the key issue. But note that for the opponent to be unprepared, they have to not see it coming. So it has to be done with a quick jab from close range.
Also, heads and such have more inertia than soccer balls so, like in karate, the blow has to be set to stop right after impact. “Follow thru” would be a really bad idea.
To this should have been added, “and you know the right WAY to hit him.”
BTW, Robby, you sound really, really snide there in your two replies to Texican’s post. Just saying.
In any event Tex was talking about damage, not impact force.
First of all, Bandit, it doesn’t matter how you hit the other guy. Whatever force you exert on him is exerted back on you. It is physically impossible to “hit the other guy harder than he hits you.” That was the point of my post.
The post that I was responding to (not to mention yours as well) admittedly raised my ire, in its cavalier misrepresentation of the “laws of physics.” My response was indeed somewhat short. Realizing this, my second post was an attempt to rectify this somewhat, though it apparently came off sounding condescending. My apologies. (I meant what I said, though—it’s been my experience in teaching this concept that people have a hard time with it, myself included.)
Finally, I would argue that damage directly corresponds to force, for two (assumed) identical objects colliding at (assumed) identical points of impact.
The ref was nutted in the temple, a much weaker spot than forehead or even nose.
Tim