One person on Reddit wanted to know if you manage to drain all the energy from a single AA battery in a single go, and translate that energy to a punch, what kind of damage would the punch leave. The answer may surprise you. Check it out after the jump.
The top-rated answer went thus:
Let’s assume we’re not using mass-energy equivalence here (i.e. we’re not obliterating the battery’s mass into pure energy). We’re just using electrical energy.
[1] This site claims that an alkaline AA battery has 9360 Joules of energy.
9360 Joules is 9360 Newton-meters. Let’s assume the punch is thrown over the distance of one meter (it’s a roundhouse with a big windup).
So, what does 9360 Newtons do to a fist over one meter? First, that energy has to move the whole arm (disregard body movement). [2] This site has an average human arm weight of 3.216kg.
Using F=ma, with F=9360N, and m=3.216kg, we find that the acceleration of your arm is 2910 meters per second squared.
Plugging this into the [3] formula for uniform acceleration (with an initial velocity of zero), we arrive at the figure that the fist hits you at approximately 76 meters per second, or 170mph.
I conclude it will hurt like a bitch, QED.
Pretty awesome physics lesson, by then another Redditor took the answer a little further:
Let’s keep going:
I’m looking at my fist, it appears to be about 3inx2in. Thus an area of ~6 square inches. That converts to 0.004 square meters. Pressure is force divided by area. Thus there are 2.3 million Pascal impacting your head. That is 339 pounds per square inch. According to my google-fu (admittedly, I found no good sources or I would like them here. If anyone has something better, please let me know) it takes ~36 psi to fracture your skull. Thus nearly 10x the pressure necessary to fracture your skull.
How about brain damage? Well, again according to answers.com, a human head has approx 5 kg of mass. Approximating the head as a uniform mass sphere, and knowing it will rotate about the neck and not the center of mass, that gives a moment of inertia (I) of 0.65mr2. The radius of a head is ~.15 m. We know F = I(alpha) + ma. Since I = .64mr2, alpha = a/r we can then solve for a = F/m(cr+1) which gives us an acceleration of ~1700 m/s2, or 174g. The human dies at any acceleration over 40g.
So yes, assuming that you are able to get all the energy out of a AA battery, put it directly into a face punching machine- the person would most assuredly die. You would puncture the skull, turn the brain to mush and most likely remove the head from the neck.
Yes, it would hurt. A lot. Our advice? Don’t try this sort of thing at home. And stay away from people brandishing AA batteries: they’re deadly.
[via Reddit.com]