This is a very thoughtful and interesting peek into how physics apply to martial arts. I’ve added this young scientist’s book to my Kindle reading list. A scientific approach can reveal much about combat sports, and football.
The range of available human motion is the dictating factor there. It’s not the laws of physics that say you have to have either a high momentum or a high energy strike. If you’re going to throw a punch you can have your muscles very loose, and in that case your body acts more like a bunch of loosely connected separate objects, but you can move very quickly. That puts very little mass behind it, but it can move very quickly. That ends up being a high energy strike, and it can cause localized tissue damage, something like that. It can hurt, but it’s not going to knock anybody back, or rotate anybody’s head and knock them out.
On the other end of the spectrum is if you actually tighten your muscles, or selectively tighten them, you can actually be one solid object at the moment of impact, and have a lot of your mass effectively behind that punch. That can knock an opponent back, or rotate their head and knock them out. So you end up with this dichotomy between the two, and those things usually don’t end up being head to head, but they are for fighting.