How dense would something have to be to stop a bullet?

  • Thread starter samsanof
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  • #1
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Let's say that an object is 3 inches thick, how dense would it have to be to stop a bullet travelling 680 mp/h?
 

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  • #2
Pythagorean
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How big is the bullet? What shape is it? Does the material have a grainy structure or a homogenous gelatin structure?
 
  • #3
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kevlar isn't particularly dense, and it will easily stop a bullet. Why do you think density is a good indicator of a material's ability to stop a bullet?
 
  • #4
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Really,what i mean is how compressed would air have to be to stop a bullet?
 
  • #5
BruceW
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that would be pretty cool. uh... If you know the range of a weapon, then you could work out the mass of air it travelled through. And then you can calculate the equivalent density of air which would give the same mass over a different volume.

But this would probably not work very well, since the air resistance forces are going to be non-linear for sure. Maybe assuming a force which goes as velocity squared would be better.
 
  • #6
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Really,what i mean is how compressed would air have to be to stop a bullet?
Not compressed at all. Our atmosphere can stop a bullet, it just takes a while. You need more information to answer this kind of thing. Do you want to stop it in a mile? In an inch? In a nanometer?
 
  • #7
Baluncore
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High density material employs conservation of energy and conservation of momentum to spread the impact load. The bullet fragments becomes part of the high density material.

A bullet delivers a high kinetic energy to a small area on the target over a very short period of time. To stop a bullet you must quickly convert that KE to heat, and / or spread the material over a much greater area. Some bullets are designed to focus their energy. Although they will be vaporised on impact there will still be a significant KE in that vapour. The vapour may be toxic.
 
  • #8
Ryan_m_b
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Not compressed at all. Our atmosphere can stop a bullet, it just takes a while. You need more information to answer this kind of thing. Do you want to stop it in a mile? In an inch? In a nanometer?
He kinda was...
Let's say that an object is 3 inches thick, how dense would it have to be to stop a bullet travelling 680 mp/h?
 
  • #9
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Ahh, my bad.
 
  • #10
UltrafastPED
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Stopping a bullet requires two things:
1. The energy of the bullet must be expended
2. The momentum of the bullet must be absorbed

A bale of cotton will do both. So will a pile of dirt.
 
  • #11
Baluncore
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samsanof said:
Really,what i mean is how compressed would air have to be to stop a bullet
The bullet will decelerate but never quite stop. The profile and weight of the bullet will make a big difference. You must redefine “stop” and provide some idea of the distance that is available to decelerate the bullet to a “safe” speed.
 
  • #12
CWatters
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If it's of interest you could estimate the acceleration the bullet must undertake in order to stop in 3 inches. If we assume constant acceleration (which might not be a valid assumption) then we can use SUVAT equation...

V2= U2 + 2as

where

V = final velocity (eg 0)
U = initial velocity (304 meters/sec)
s = distance (eg 3 inches = 0.075 meters)
a = acceleration.

Rearranging the equation gives

a= (V2 - U2)/2s

a = -3042/0.150
= -62,000 ms-2

Roughly 6200g
 
  • #13
CWatters
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Really,what i mean is how compressed would air have to be to stop a bullet?
I suspect a bullet would pass through 75mm of liquid nitrogen (at atmospheric pressure but that's not quite what you are asking).
 
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