Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Body armor

  1. Oct 16, 2008 #1
    hallo all, I was wondering if you were able to help me with the following:

    I am wondering about how body armor works exactly and have checked some sites, but still strugle with some questions.

    As for know I have the following: when a bullet hits a body, then you have injuries caused by the bullet itself (a hole in your body) but there is also the impact of the bullet on your body: the bullet has a certain amount of kinetic energy and this energy is given to your body.
    Now first question: a bullet hitting the body is this an elastic or inelastic collision ? I was thinking that when the bullet leaves the body its inelastic and when the bullet stays in the body its elastic.

    Now the body absorbs some energy, but how can I state this?
    I was thinking that the impact of the bullet causes the bullet to "give" his kinetic energy to the body by causing waves to occure, these waves, wich are actually Oscillations , causes injuries in the body as the body can only take a certain amount of oscillations, energy.

    I wonder how I can explain the wave and Oscillation story better.

    then I wonder about the following thing: a body armor will try to spread the energy of the bullet , so that the body can cope with the energy (waves) but what if you fire 2 times very fast ? Does interference occur then? I mean: positive interference because you have 2 waves very shorty after each other, meaning they will be added up and thus you have a wave that is a lot bigger?


    thanks in advance.
  2. jcsd
  3. Oct 16, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper

    The collision would be elastic if no energy was lost by the system = bullet+person generally bullet impacts are not elastic - energy is lost to melting the bullet etc.
    If the bullet goes through or not changes the amount of energy transferred to the person from the bullet - this is the important thing.

    The point of the body armour is to stop the bullet penetrating (=bad) and to spread the force over a larger area. The KE of the bullet isn't actually that large. eg 15g bullet at 575m/s = 2500J
  4. Oct 16, 2008 #3


    User Avatar
    Science Advisor

    There is an http://www.metalstorm.com/content/view/64/109/" [Broken]which is designed to do exactly that. It fires two bullets so quickly that the second exits the muzzle before recoil has made the muzzle move, so both bullets will hit the exact same spot. If this hits any existing body armour, what actually happens is that the first bullet deforms the armour to its limit, and the second one breaks through.
    Last edited by a moderator: May 3, 2017
  5. Oct 16, 2008 #4

    Someone told me that when you hit someone 2 times very fast in a row: the person will die just because you have positive interference and the waves will strengthen eachother causing leathel injuries.
    the body will not be able to cope with such wave, vibration.

    ok , but still: is this based on interference? or simply based on: deforming the armour and making it possible that the other can pass true?

    And what if there would be no body armor and imagine that the bullet itself would not be leathel==> what if you shoot so quick that the bullets hit the person almost at the exact same time, causing a huge amount of kinetic energy + positive interference : the shockwaves will be bigger because of the interference.

  6. Oct 16, 2008 #5


    User Avatar
    Science Advisor
    Homework Helper

    It is possible, high velocity rounds cause compression/expansion pressure waves in the body. As they tavel through the bag of water that is a perso the waves rebound from the otherside ( there is a big impedance mismatch betwene you and the air) it is possible that the incoming shock wave from the next round (if timed correctly) could combine with this.

    Some armour piercing rounds contain two parts for a different reason. Tanks have reative armour which contains a layer of explosive that detonates to deflect an incoming round. The two part rounds have a front part that detonates this and then a second slower round that penetrates the remaining armour. Some high rate of fire guns like that in the A10 aircraft do this by firing a bunch of rounds very close together.
  7. Oct 21, 2008 #6
    coming back on the reactive tank armor:

    could you say that the explosives on the tank react as a sort of negative interference? You have the shockwave of the granate (impact) and you have the shockwave of the explosives. They work against each other, the explosives on the tank are there to "kick away" the explosive shockwave of the incoming granate.
    I know you also make sure that the head of the granate explodes before it can do major damages , but if you look at it very simply then you have in a way to waves working against eachother?

    or not?
  8. Oct 21, 2008 #7


    User Avatar
    Science Advisor
    Homework Helper

    No the blast simply deflects the incoming penetrator armour peircing round. You can't efficently couple much power from explosives into air.
  9. Oct 22, 2008 #8


    User Avatar
    Science Advisor

    Right. Armor-piercing antitank rounds are made to punch through plating for a very short distance, then the force disperses quickly. Reactive armor causes the round to detonate early, penetrating an obstical several inches or a foot form the surface of the tank, then disperses its force before reaching that surface.

    As for constructive interference in a human body; as mgb_phys mentioned, the timing would have to be perfect. Its not just a matter of hitting twice within a certain amount of time, but precisely at the right time. If the second round hits 1/2 wavelength behind the first, the interference will actually be destructive, causing a smaller shockwave than the bullet would have generated if it had arrived alone.

    But 'm not sure how the extra energy would act within the body.
  10. Oct 25, 2008 #9
    In a sudden-kill scenario there are only three choices... change your position relative to the impact, change the trajectory of the incoming round(s), or lessen the impact result.

    Either choices are damn hard to do in the "field", but that's what we have.
  11. Oct 25, 2008 #10
    With IED's potentializing at 7,500 mph (using tungsten disc's) this is a real problem.
  12. Oct 25, 2008 #11
    I wish I had an answer, but I don't.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook