|MadSci Network: Physics|
Joseph, Theyíre great stories arenít they? A bible, medal, wallet, coin, or some other object saves a soldier. Is it possible? What kills you when you are shot? Most likely you bleed to death internally when an artery is severed. To prevent that you must keep the bullet from entering your body. Obviously sitting in a tank keeps the bullet from entering your body because the tankís armor stops the bullet. What about body armor? The key is to spread the energy of impact over a large enough area so that no one point receives enough pressure to penetrate the body. This will work if the bullet does not go through a gap in the armor or have enough energy to break through. Thus body armor can protect you from shrapnel and small arms fire. But what about a small object? Letís try a coin. Letís hit a US Quarter with an 8 mm round. To make life easier lets say the bullet is 1 cm in diameter and the quarter is 3 cm. How much would the pressure be reduced by hitting the quarter. The pressure would not be 1/3 as great it would be 1/9th. The area of a circle is of course related to radius squared. Thus tripling the diameter multiples the area by nine and reduces the pressure to 1/9th. Would this be enough? Unless the bullet was nearly spent and had slowed down considerably, you are still in big trouble. So if the bullet hits the quarter square on you are probably toast. But what if it strikes it at a shallow angle? Could the quarter save you? Let us say that a bullet hits the quarter at a thirty degree angle. The quarter already reduces the force by 1/9th and the oblique angle reduces it further. Let us assume the bullet ricocheted off at 30 degrees the other way. The force of impact would be mv/t. And since the velocity normal to the surface is changed from Ĺ v one way to Ĺ v the other you still receive quite a jolt as you receive an impulse of mv. In fact the coin might do more harm then good. The impulse and change in momentum the bullet receives is equal and opposite that received by the quarter. Thus the quarter may be driven into you! This large irregularly shaped object would be quite capable of slicing some arteries. The Mythbusters TV show did a segment on this a while back. You may be able to check their archives. http://dsc.discovery.com/fansites/mythbusters/mythbusters.html In the end it depends on the velocity of the bullet, the angle it strikes the surface, and; most importantly, is there anything to spread the force over a greater area. If I were you I would use the quarters on video machines and stay out of war zones. Ed firstname.lastname@example.org
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