MadSci Network: Physics

Re: Can a small metal object stop a bullet and prevent a person from being hit?

Date: Sat Apr 2 06:15:15 2005
Posted By: Ed Stammel, Faculty, Computer, SUNY Delhi
Area of science: Physics
ID: 1112230576.Ph


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.

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.


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