Question about calculating bullet penetration

[MrBarton]

I'm trying to disprove the movie Wanted for a less than obvious reason. In the final scene a .45 ACP round travels through 11 human heads. I was curious of how I could mathematically disprove this being possible. Would I use the energy of the bullet or the force? and with either, how would I go about calculating. Thanks alot!

 

[madmike159]

Well you would need to know what velocity a .45 ACP round travles at, and also how much resistive force a human head would provide. It should be obvious that it couldn't anyway.

 

[QuantumPion]

You can probably find on the internet somewhere the average penetration of a .45 ball round in gelatin. My guess is that it would be somewhere on the order of 12-24" of gel. This doesn't take into account the hardness of bone or the disintegration of the bullet itself so that would be an upper limit.

 

[rcgldr]

The fact that bullets curve significantly in the movie should be a clue it's far from real as ghosts that go through walls but walk on floors. I doubt a pistol could travel through two bodies, not even a desert eagle. On the other hand, the 30 mm rounds from Apache (chain gun) or A-10 (mini-gu) could do it:

http://en.wikipedia.org/wiki/File:30_mm.jpg

 

[madmike159]

I once saw a video of a 50 cal sniper round going through a bullet proff vest, through a water filled dummy (similar to a human torso) and out the other side. Even so I doubt that would go through many heads, maybe 4.

 

[MrBarton]

Well I know that it is impossible for bullets to take that trajectory and such (obviously), but in terms of physics, mathematically, I was wanting to know how I would go about disproving that a bullet wouldn't go through 11 heads. I'm not sure If I'd use the energy of the bullet, or the force of the bullet. The acceleration (deceleration) of a bullet hitting a skull is unknown, so i figure energy. I was curious to know how I would find out how much bone, or brain matter would take energy from the bullet.

 

[diazona]

I think the Mythbusters might have done this, or something like it, once... they did show that 14 pizza boxes in thermally insulated carrying cases will stop a bullet ;-) But seriously, I think the only really believable way to disprove it would be to build a bunch of fake human heads and shoot them.

If I had to calculate it I'd probably try energy, but at some point you would need to figure out how much energy is dissipated when a bullet passes through a head, and I have no idea how you'd go about calculating that from first principles (i.e. without looking up some experimental parameter at some point). Heads are very complicated objects.

 

[madmike159]

What you need to do is find how much force a head provides (per meter) against a bullet. Then you can work out how much energy is lost by passing through each head. You will also need the velocity and mass of the bullet as it leaves the barrel. Even with all this it would still be unaccurate because the bullet would deform.

 

[johnsmi]

Nice,
I'd like to know, using momentums (momenta) how fast you'd have to spin the pistol round in order for the bullet to travel in a roughly 2m radius :rofl:

 

[captn]

I'd like to know... how fast you'd have to spin the pistol round in order for the bullet to travel in a roughly 2m radius

I assume, by spin, you mean the shooter rotating as (s)he pulls the trigger?

Doing so will give the bullet some lateral velocity, but once the bullet leaves the barrel, it will travel in a straight line-- not in a circle (and not in the direction of the bullet axis.)

Neil

 

[madmike159]

You can't curve a bullet, because the barrel is straight.