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Assume it's a standard .45 caliber handgun. Shooter is 10 feet away, and we hold a desk, mini-table, blackboard, etc on our chests(to protect vitals).

How do you figure out the bullet's decrease in velocity, and likelihood of flying away from you.

I got the idea thinking whether could we protect ourselves against a madman like the VT or Columbine guys?

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try include more details into your question. –  bubble Jul 14 '11 at 20:14
    
OK sure, will do. Thank You, –  Adel Jul 14 '11 at 20:16
    
As is this can be answered with a simple yes or no, which isn't very useful. (Putting in numbers doesn't help that much.) It would be better to ask how to figure it out. –  David Z Jul 14 '11 at 20:24
    
@David - that's a good point. I'm still a newbie. I edited it to be more quantifiable :) –  Adel Jul 14 '11 at 20:25
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@Adel: your effort is appreciated :-) FWIW, great questions don't always have to be quantitative (although they often are). If I were asking this, I would title it something like, "Is it possible for a table to deflect or stop a bullet?" and then in the body, ask about how the bullet speed, angle of impact, etc. would affect the situation. That would invite an answer that explains the physics of inelastic collisions. I'm not saying you have to (or should) change your question to say that, I'm just trying to help you get a sense of what constitutes a well-posed question around here. –  David Z Jul 14 '11 at 22:43
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up vote 4 down vote accepted

In order to answer this question we must make some more assumptions. Given your scenario, one of the tragic school shootings which occurred in the States, we can assume the shooter will be within 50 feet. Since we are at a school, then let us assume typical materials found there such as desks and tables composed of laminate or other glued wooden materials augmented by plastic. With these assumptions, any bullet from a .22 caliber (~ 2 oz) and larger at close range will most likely penetrate the barrier.

When a bullet impacts upon a surface of a material as given above, it penetrates, begins to deform and may fragment in the material. To determine how it slows and deforms, we must consider the size, shape, velocity and composition of the bullet, the interaction of the bullet with atmosphere, such as its dynamic stability, as well as the material response of the material being hit. Laminates should exhibit an inelastic response to the bullet adding to the difficulty of a priori calculating what will happen besides simply saying that it will go through or not.

Most bullets are made of lead surrounded (jacketed fully or partially) by a harder metal such as copper. This is done as a trade off between ballistic stability and deformability in order to wound. There are several types of bullets, the 2 most common are full metal jacket bullets, which are conical at the tip and various hollow tip styles which are blunt tipped and designed to expand as they penetrate the material since the segmented jacket peels back at it travels through the material creating a larger wound channel due to the high pressure zone commonly associated with hollow-tip bullets.

A bullets decrease in velocity due to traveling through a material is a nontrivial calculation which is usually not even attempted. Typically, the velocity given is an average muzzle velocity and the energy in foot pounds is given as the total amount of energy dumped into a ballistic pendulum, at a given distance, exhibiting an elastic impulsive response to the collision. The bullet hits it, the pendulum moves, and knowing facts about the bullet and pendulum, we calculate the energy deposited in the collision as an average.

Ballistics is an experimental science, in which bullets are created according to various known criteria and then tested against various barriers while measuring performance before and after penetration. For handguns, after a certain point, bigger is not always better and jacketed blunt bullets (hollow tips) can cause a more lethal wound than a full metal jacket despite the fact that they may not penetrate as far in ballistic gelatin. It is possible that bullets of different sizes but traveling at varying speeds will give similar wound profiles.

A .45 caliber bullet is capable of penetrating a typical hollow cinderblock if it is jacketed. Given what you present, desks or tables provide little cover, but better than none at all. Real examples of ballistics tests can be found here:

http://www.ballisticsbytheinch.com/

http://www.theboxotruth.com/docs/bot1.htm

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Thank You so much - excellent answer! –  Adel Jul 14 '11 at 21:40
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I wouldn't say calculation isn't attempted. It is done via nonlinear finite element analysis. A lot of armour anti-armour work is done, both experimentally, and via nonlinear dynamics computer codes. Sometimes a projectile will turn around and be reflected. –  Omega Centauri Jul 14 '11 at 22:51
    
@Omega, that's fascinating - it can even turn right back at the shooter. –  Adel Jul 14 '11 at 22:55
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I should clarify. Omega is right-- for some applications FE calculations are done. I was referring more to arbitrary handgun cartridges impinging heterogenous media, such as laminates. AFAIK the published FE simulations usually are bullet/penetrator on armor plate etc for military reasons. Interestingly, some rifle bullets can acquire MORE penetrating power after initial deformation through a metal surface. These kinds of observations are difficult to explain unless you run the FE calculations to visualize what is occuring. Appendix references here: mate.tue.nl/mate/pdfs/6290.pdf –  dergeophysiker Jul 14 '11 at 23:56
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There are also important areas of balsitic-like analysis that are nonmilitary. Foam or ice versus the space shuttle got a lot of attention. Bird strikes on aircraft are also important, although bird strikes on engines are greatly complicated by the interaction with the fan blades. –  Omega Centauri Jul 15 '11 at 15:58
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