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Projectile weapon-physics

  1. Oct 16, 2003 #1
    [SOLVED] projectile weapon-physics

    Are really revolvers, shotguns and rifles powerful enough to toss people backwards? Or is this an image created by Hollywood films?

    Thanks for any answers concidering this ;)
     
  2. jcsd
  3. Oct 16, 2003 #2

    Njorl

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    A .44 magnum will fire a 15.5 g bullet at about 450 m/s. This would impart enough momentum to accelerate an 80 kg man to 8.7 cm/sec, if it doesn't fly through him. A high-powered rifle will have 2/3 the mass but twice the velocity, imparting a velocity of about 12 cm/sec. A 12-gauge shotgun will fire about 35 g of shot at about 280 m/s - roughly the same momentum as the rifle.

    I don't think these velocities are going to send people flying, but there will be knockback.

    Njorl
     
  4. Oct 16, 2003 #3

    LURCH

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    I saw a demonstration on one of those "bloopers" programs. A news reporter was talking about a particular police precinct in which officers would soon begin wearing bulletproof vests (the kind with a metal plates, before Kevlar). He closed his report with the words, "it may be somewhat cumbersome, but in an emergency this(holds up one of the metal plates) could save and officers life." At this point, a revolver (which looked like a .38) points in from offscreen at the plate, which is held in front of the reporter's chest.

    "Dirk Witless, Channel 14 action news, reporting." And the weapon was discharged. The reporter's feet were lifted off the ground and he flew backward (just a couple of feet) and landed on his back. What followed was a long series of bleeps, with the words, "that HURT!" somewhere in the middle.
     
  5. Oct 16, 2003 #4

    turin

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    A bullet CAN knock you down.
     
  6. Oct 16, 2003 #5
    I have heard somewhat the opposite in reference to the Kennedy assasination. JFK's head was supposed to have jerked back toward the bullet impact (no grassy knoll). Of course for momentum conservation there had to be brain matter moving forward. Sorry, Jack.
     
  7. Oct 17, 2003 #6

    Njorl

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    With JFK, the theory is that the motion was due to the part of the brain hit. There was not much momentum transfer as the bullet passed through with high enough velocity to pass through the front seat and governer Connelly.

    Njorl
     
  8. Oct 17, 2003 #7
    Have you ever seen a lightweight after they fire a shotgun the first time? It is not unusual for them to end up on their can.
     
  9. Oct 17, 2003 #8
    I heard this was due to the "jet effect" that happens as brain matter follows the bullet at high speed due to vacuum pressure, producing a noticable thrust backward.
     
  10. Oct 17, 2003 #9

    russ_watters

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    Skeet shooting with the Boy Scouts. A friend of mine (12 years old but probably 150 pounds) though he was too cool to stand sideways and got knocked over the first time he fired a 12 gage.

    The thing that surprised me when I first fired one from the hip is that it does not jump out of your hands. It can be a lot of force if you try to stop it cold (against your shoulder) but if there is some travel, it is easier to absorb the energy.
     
  11. Oct 17, 2003 #10

    krab

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    This is a good point which did not immediately occur to me (gettin slow). If a body absorbs the impact of a bullet, the momentum transferred is exactly the same (neglecting air resistance) as the impulse of the kickback when the gun is fired. Ergo, if the kickback can put an unprepared person on his can, so can the shot. In fact the recipient is much less likely to be able to brace for impact and so could easily end up falling over.
     
  12. Oct 17, 2003 #11

    Chi Meson

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    Hollywood's worse when they show no kickback.

    Remember in Eraser, when Governor Schwarzenegger is hanging in a parachute and he fires a gun at the oncoming airplane and doesn't budge an inch? Or when Thelma and Louise fire off several rounds into the tanker truck (which blows up) without any kickback at all? This the the fiction. Getting knocked over is real.
     
  13. Oct 18, 2003 #12
    Alternative to Njorl's explanation, think of this. If the momentum of the bullet can send its target flying, then, according to the principle conservation of momentum, the gun/rifle, and hence the shooter, will also experience that same amount of momentum in the opposite direction. So in actual fact, if the target flies off after biting a bullet, so must the firer.
     
  14. Oct 18, 2003 #13
    but the shooter should be in a strong steady position and prepared to fire. the target is probably not ready for it and not in a very strong stance. the shooter can absorb recoil by the positioning of their body.
     
  15. Oct 18, 2003 #14

    LURCH

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    Right. Exactly the way one man can punch another and knock him down, without knocking himself down in the process.
     
  16. Oct 18, 2003 #15
    Even if the shooter is in a prepared position, the force acting on him due to the recoil will still be great... it'd be as if catching hold of the 'flying' target. I personally believe it is quite impossible to remain in a position to continue to fire if I've just caught hold of a person being shot by a Hollywood bullet.
     
  17. Oct 19, 2003 #16

    Njorl

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    The difference between what happens to the shooter and the shot is often a matter of angular, rather than linear momentum. The shooter, with one foot ahead of the other and weight well distributed, will have his weight rock onto the back foot. The shot guy, unprepared, may well have his feet positioned in a line perpendicular to the flight of the bullet. If his feet don't move, he will be whipped into the ground.

    Njorl
     
  18. Oct 19, 2003 #17

    russ_watters

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    Military assault rifles used to be fully automatic, but they have switched to 3 round burst because even with a counterweight to absorb much of the recoil, they are virtually impossible to hold steady. Watching someone try to fire an old M-14 in fully-auto mode is something to see.
     
  19. Jul 27, 2006 #18
    What if.

    Haven't any of you seen the actual physics in action? No way would a person be sent back five feet by a .50 caliber projectile, no way.

    I have a question for you guys. Do you know what the impact is of a .45 caliber bullet or a 5.56 nato round? I'm curious. At school they didn't teach us the formula for the amount of impact for a regular bullet, so I'm curious. How many pounds of force can a bullet inflict if going at a supersonic speed?:confused:
     
  20. Jul 27, 2006 #19

    Danger

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    Hi, Mr. Mad. You can find all of the ballistics information that you want from the ammunition manufacturers or in various gun magazines. My .44 magnum Super Blackhawk produces 1,150 ft/lbs of muzzle energy from a 7 1/2" barrel. Strangely enough, both that and the mid-range ballistics are identical to a .30-30 Winchester fired from the same barrel length.
     
  21. Jul 27, 2006 #20

    russ_watters

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    Pounds of force is entirely dependent on the way the impact is absorbed and is largely irrelevant anyway. The relevant issue is momentum. Momentum is just mv.

    Regarding knocking someone down, I didn't mean to imply a bullet can knock someone back much. If you stand with your feet square to where you are shooting, your center of gravity is only a couple of inches in from the back of your heel. That means the shotgun only needs to knock you back a few inches to knock you over.
     
    Last edited: Jul 27, 2006
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