Physics Help Forum Bullet Deflection Question
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 Aug 17th 2010, 05:50 PM #1 Junior Member   Join Date: Aug 2010 Posts: 2 Bullet Deflection Question Hello Mighty Ones! I know from an FBI study that a bullet shot at a 90 degree angle into a metal plate, the splatter from the bullet goes out at a 20 degree angle. The question is... is there a rule or something that says if I angle that 90 degrees to angle down X degrees will the deflection result in a major loss in energy that the plate has to absorb. Now I'm trying to sound all smart and we both know better I'm just trying to give a good explanation So bottom line is. Were making a steel target to shoot with the front of the target angled to help not only in deflection of the bullet but also to save wear on the target itself. Is there a diminishing return on the angle once it gets to a certain point on how much it will help in energy absorption and what is that angle? Thanks, FHKBaker
 Aug 17th 2010, 10:34 PM #2 Junior Member   Join Date: Aug 2010 Posts: 2 Here is a response from a friend now I just need to ask him to decipher it The amount of energy the plate has to absorb depends only on the initial and final speeds of the bullet. If the bullet completely stops, then the plate has to absorb 1/2 * mass * speed ^2 (speed squared, or speed * speed). Now, if the bullet comes in at a glancing blow, at some angle other than 90, then the bullet will leave with some speed, so the amount of energy the plate absorbs is smaller. The question is, what is the final speed of the bullet? That's not a simple question to answer. One assumption, or approximation, is that the bullet loses all the speed perpendicular to the plate, but keeps all the speed parallel to the plate. In that case, the speed after the impact is v*cos(angle), where v is the initial speed. So, the amount of energy the plate must absorb is given by: 1/2 * mass * initial speed^2 *( 1 - cos^2(angle) ) When the angle is 90 degrees, the amount of energy absorbed is all the energy in the bullet. When the angle is 45 degrees, the amount of energy absorbed is half the initial energy of the bullet. This assumes that the plate absorbs all the momentum perpendicular to the plate, but the bullet keeps all the momentum parallel to the plate. I don't know if that's a good assumption or not. You can plot 1 - cos^2(theta) to see what the curve looks like as a function of the angle theta.

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