WARNING! THE FOLLOWING IS APPROACHING CRITICAL NERD. DO NOT CONTINUE IF YOU WISH TO REMAIN COOL.
To whom it may concern:
I think momentum is the much more germane calculation/nerd factor for striking than kinetic energy. First of all it is a vector so the direction of the force matters. Second, and this is my opinion based on some text book/lab understanding of physics/and mechanisms of wounding combined with redneck/dumb guy normal thought, it better describes what we know happens when we punch or get punched.
Momentum, p, is simply mass, expressed in kg, multiplied by velocity, expressed in meters per second. p = mv expressed in kg-m/s (kilogram meters per second). Kinetic energy undervalues mass and way oversells velocity when we are talking about punches/kicks/strikes. I grant that KE might have more sway when discussing high velocity (lets say 2000 ft/sec plus) rifle bullets because cavitation in tissue is a mechanism of wounding, but with punches momentum tells a better story.
If KE was the appropriate measure, than holding a small weight in my hand, e.g. a roll of quarters, would make me hit lighter than without it. My fist would be traveling slower (lets assume similar punch mechanics so straight vs straight). The velocity difference would be an exponential factor.
This doesn't seem to work out. The hand load makes you hit harder. Not that it is at all necessary to do physics calculations in order to knock someone onto queer street.
A salient point with both p and KE is that they use velocity not speed. Velocity is directional, so only the amount of ass, mass, you get going in the direction of the strike, velocity, matters.
Again good posts guys.
I recommend getting a copy of Jack Dempsey's book if anyone reading this is interested in the mechanics of punching.