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Bullet question

  1. Apr 9, 2006 #1
    Me and my friends were arguing if a bullet can push a person back. I'm the only one that thinks it won't push a person back even if it doesn't go through simply because the force is not big enough to push a person back.
    Also, say a person was coated with a layer of diamonds, and was shot in the chest. Would the person fly back a good distance?
  2. jcsd
  3. Apr 9, 2006 #2
    Let's take a bullet traveling at a 1000 m/s and weighing about 100g, I don't know much about bullets but I would guess this would be pretty fast. Let's also assume there's no friction and the bullet gets stuck in a man weighing about 70kg. Then the man and bullet after the collision will have a speed of about (.1*1000)/(.1+70)= 1.43 m/s which is about less than 3 mph, this is assuming there's no friction. My guess is in an actual situation depending on the bullet the man would either not move at all or maybe take a step back...assuming he wasn't shot in the legs :). Didn't mythbusters have an episode about this, I think they did?
  4. Apr 9, 2006 #3


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    A bullet traveling at 1000 m/s is one fast bullet, that is almost 3 times the speed of sound at sea level. Also, I think the "buisness end" of the bullet, the part that actually flies through the air and hits the object, is considerably less massive than 100 grams...most of the bullet is casing and propellent which stays behind in the gun.
    But using these over estimates, the law of conservation of momentum tells us just how effective a bullet would be at knocking someone over.

    Another way to think of it would be in terms of the energy the bullet delivers.
    Take the .50 BMG bullet, this thing is gigantic.
    According to Wikipedia, this bullet can deliver somewhere from 10,000 to 13,000 Foot-Pounds-[force?] of energy which converts to somewhere in between 13.6 kJ and 17.6 kJ of energy.
    If we assume the mass of the bullet is negligible copared to the mass of the 70 kg man and if this bullet deilvers its full 17.6 kJ of energy into the person [at rest], chaning into KE, the person would have a final velocity of 22.4 m/s, which is about 50 miles per hour, this seems like a lot, but just look at this thing, it is 10 cm long not counting the "slug",
    http://upload.wikimedia.org/wikipedia/en/2/22/50bmg.jpg [Broken]
    This of course assumes 100% efficency at transfering energy from the bullet into KE of the person.
    Last edited by a moderator: May 2, 2017
  5. Apr 9, 2006 #4
    Except that a .50 will never stay in a person.....

    a .50 usually goes around 900m/s. The bullet's mass is about 550 grams which equals just below 36 grams. Therefore, the kinetic energy is 14580J. I guess wikipedia was correct! Using conservation of momentum, it can be calculated that if somehow a human with mass of 60kg were to stop the bullet, the human's velocity would be just about .53 m/s. I have no clue where you came up with your value???? If it has an inelastic collision, KE will not be conserved. Another way to think about it is this, if it had, for example, .2 meters to stop inside a body, it would exert an average force of 72900N (wow). Using this value, once again, you can calculate the human velocity to be .53 m/s(considering that the same method as earlier was used, just about).

    Anyway, a lot of kinetic energy is lost in this... However, if the bullet were to bounce off the human in a perfectly elastic collision (lol), then the person would travel at 1.08m/s, and the bullet would go back towards the shooter at 898.9m/s.

    No matter how you look at it, the kinetic energy of the bullet can't all be transfered into the human's kinetic energy.
  6. Apr 9, 2006 #5


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    They even have done this on the American television show Mythbusters. They shot a pig carcass with a variety of guns including pistols, rifles, sub machine guns, shotguns, and they had like ten people shot at the same time. The carcass did not move significantly like in the movies.
  7. Apr 10, 2006 #6


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    I saw the Mythbuster's ep too. While the show is suspect when it comes to rigorous control of conditions in their experiments, Mythbusters does have the virtue of being an empirical source of data.

    The pig was suspended on a hair-trigger mechanism so that the slightest movement would cause it to fall. No matter how many times they shot at it, they could not even get it to move enough to come off its mount. It barely moved at all.
  8. Apr 10, 2006 #7

    How far away were they standing?

    No, I do believe that a point blank blast from a higher caliber gun will cause someting to move backwards
  9. Apr 10, 2006 #8


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    Another way to look at it empirically, without involving math, is just the basic action/reaction relationship. If the recoil of the gun doesn't knock the shooter over, there's no reason to expect the victim to be thrown anywhere. Most shooting victims, in fact, tend to fall toward the source of the shot.
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