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

  1. Aug 31, 2013 #1
    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?
  2. jcsd
  3. Aug 31, 2013 #2


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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?
  4. Aug 31, 2013 #3
    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?
  5. Aug 31, 2013 #4
    Really,what i mean is how compressed would air have to be to stop a bullet?
  6. Sep 1, 2013 #5


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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.
  7. Sep 1, 2013 #6
    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?
  8. Sep 1, 2013 #7


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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.
  9. Sep 1, 2013 #8


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    He kinda was...
  10. Sep 2, 2013 #9
    Ahh, my bad.
  11. Sep 2, 2013 #10


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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.
  12. Sep 2, 2013 #11


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    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.
  13. Sep 2, 2013 #12


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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


    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
  14. Sep 2, 2013 #13


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    I suspect a bullet would pass through 75mm of liquid nitrogen (at atmospheric pressure but that's not quite what you are asking).
    Last edited: Sep 2, 2013
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