View Full Version : a question for any physics majors or physicists out there.

04-22-2010, 09:45 AM
Quick question. How does the rotational momentum of a bullet compare to its translational momentum?

The rotational velocity is about 9 orders of magnitude faster, but my rotational dynamics knowledge is junk (especially concerning momentum) so I don't know how this translates to momentum (or kinetic energy).

Basically, when a bullet penetrates a material (polyethylene for example), how does the transfer of momentum compare between the translational motion and the rotational motion?

If need some numbers (sorry for the units, but I am doing this on a molecular scale)

1.14E+7 ang/fs translational and 2.70E+16 ang/fs rotational (about 250,000 rpms).

Assume a 5.7 mm bullet.

Thanks a ton if you answer this.

04-26-2010, 07:13 PM
Are you talking about centrifugal forces here? As in how much energy is needed/used to rip the bullet apart after it is misshapen??

I'm a geek and know weapons, but I'm confusilated here...

04-27-2010, 07:52 AM
I think I resolved it. But I'll shed some light, in case you have some contradicting evidence.

I am wondering how much momentum is transferred when a bullet passes through a piece of plastic. When it punctures the material, is the majority of the momentum transferred in the translational direction, or is there sufficient transfer of momentum on the border of the bullet hole.

In detail, I am a molecular modeler who is trying to see which direction to apply a velocity term for the polymer being shot at. On the molecular level, from the polymer's point of view, will it slide in the direction of the rotation or of the translation of the bullet?

From viewing the actual polymer plates that were shot at, it doesn't look like it was rotated much.

04-28-2010, 04:50 PM
Okay, I got it now.

In my opinion, the translation would be in BOTH directions... the polymer would be slightly spun in the direction of the rotation of the bullet, AND it would be pulled in the direction of linear travel. Shooting upward through a sheet of plastic, the resulting "wound" would look like a small volcano - slightly twisted, and poking up slightly on the top of the sheet of plastic. The largest transfer would be from the linear travel, as the centrifugal and centripetal forces in the bullet itself would cancel each other out, and the amount of time for transference would be extremely limited. I would be VERY surprised to see as much as a 5-degree (of a 360-degree revolution) translation in the deformity of the plastic.

This is, of course, depending up on the thickness and elasticity of the plastic itself. If it's rigid (plexiglass) and around .25" thick, I believe you'd see what I've described (I've seen bullet-riddled plexiglass). If it's thicker, the rotational translation of energy would be greater the farther the bullet remained in contact with the plastic. If the stuff is nearly fluid, you wouldn't see crap... because it would reform to near pre-shot condition after the bullet passed through or stopped moving (think ballistic gel).

Howzat for a physics guess?

04-28-2010, 04:54 PM
I have to concur.

05-03-2010, 12:20 PM
Ok, now my brain aches from trying to understand what you two are talking about.

05-05-2010, 07:34 PM
Hard, cold physics just bores me to tears - IF I can stay awake long enough to actually HEAR people discuss it. However, when physics is applied to something that I KNOW ABOUT, then I have no problem keeping a clear mind and actually enjoy the darned subject.

I took an Environmental Physics class in college (a 400-level course!) that I dreaded. The first day of class, this tiny Asian man (Dr. Ng) steps up and spoke the only full sentences I understood. (It's best said in an over-the-top Asian accent) "I am Dahctah Eng. I frahm Bietnam. I teeoretical pyzzicist foh NAHSAH. I LOB pyzzics!" The rest of the class was totally unintelligible - but it didn't matter. Thanks to blackboards and our texts, we calculated how much energy was lost in high-tension power lines from power plants to our microwave in the Kitchen... we calculated energy produced by wind and water turbines... we calculated decay rates of disposable diapers as opposed to heat and pollution produced by using cloth diapers (as disgusting as they are, disposables are better for the environment!)... but the capstone was calculating the ambient heat of the Earth produced by the maelstrom of magma within our core.

The temp of the Earth SHOULD be around -260 degrees Kelvin... Then we calculated how much heat was produced by the sun... and our temp was raised all the way up to -23 degrees Kelvin. It was up to US to figure out why it was 24 degrees Kelvin outside and all the girls were dressed in short-shorts and bikinis...

Global warming and the greenhouse effect. The only reason we are alive is due to global warming and the greenhouse effect.

I learned SO much about physics and how to apply it from that guy. He got SO excited when he brought up a new principle that it was infectious. I would love to take another class from him!!