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Question about bullet fired from gun.

  1. Mar 9, 2008 #1
    Does Bernoulli's principle come into play, regarding the fact that a bullet my travel hundreds of yards and drop very little?

    Or is gravity pulling the bullet down at the same rate it would any other object, and it's just that the bullet is in the air for only a fraction of a second?
     
  2. jcsd
  3. Mar 9, 2008 #2

    turbo

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    The bullet starts falling as soon as it leaves the barrel.
     
  4. Mar 9, 2008 #3
    Good the second. :smile:
     
  5. Mar 9, 2008 #4

    russ_watters

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    Recently in my area, a guy with little experience firing his military-grade rifle accidentally shot it into a house a half a mile from the shooting range he was using when he set the sight elevation for a 500yd range instead of a 50 yd range. The sight elevation is calculated based on a parabolic trajectory. At 1100 ft/sec, the bullet was in the air for 2.4 seconds therefore rose (and dropped) 92 feet.
     
  6. Mar 9, 2008 #5

    rcgldr

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    The spin on a bullet will keep it near horizontal, and this will make it behave like a poor glider, it's downward component of drag will cause it to fall slower than a bullet simply dropped that rotates into downward facing orientation.
     
  7. Mar 9, 2008 #6

    turbo

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    Not true!

    The sights of a rifle are arranged to make the barrel of the rifle point above the point of aim in order to compensate for gravity (which acts on the bullet as soon as the bullet leaves the barrel). In the case of a high-powered rifle with a light bullet, the difference is moderate, and in the case of a lower-powered rifle with a heavy slug (like a .38-55 for instance) the difference in barrel elevation and line-of-sight to the point of aim is quite significant. The only reason for rifling is that spinning the bullet keeps it stable, and inhibits tumbling, which reduces drag and increases accuracy. Bullets do not "fly". A bullet dropped from your hand will hit level ground at the very same time as a bullet fired out of the barrel of a gun.
     
  8. Mar 9, 2008 #7

    russ_watters

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    Actually, that's not quite right: it will hit 92 feet below the aim point. The rise and fall is half of that.
     
  9. Mar 9, 2008 #8

    rcgldr

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    I find it hard to believe that the coefficient of drag of a bullet falling on it's side is exactly the same as a bullet falling nose down.
     
  10. Mar 9, 2008 #9

    turbo

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    You might want to study ballistics. This is a very old and demanding field, and it addresses the question of gravitational influence quite precisely.
     
  11. Mar 9, 2008 #10
    Haven't you ever done the physics problem with the monkey falling out of the tree?
     
  12. Mar 10, 2008 #11
    ]a bullet that is dropped will not fall on its side, nor will it fall straight down. it will tumble through the air.

    gravity pulls the same on any object, but the force of air resisting gravity changes. It is the coefficients of drag that determine the vertical (and horizontal) force of friction from the air, and therefore how quickly it falls.

    in a vacuum a bullet fired horizontally and dropped from your hand will land at the same time, but a bullet tumbling through the air straight down will not hit the ground at the same time as a bullet flying horizontally.

    In all likelihood the bullet falling straight down will land first, since the horizontal position of the bullet that was fired straight results in the highest possible vertical coefficient of drag of any orientation. (with the only possible exception being a bullet that is falling with the flat, back end facing down, which is physically impossible since it would never be in unstable equilibrium).

    in laymans terms - the air pushing UP on the bullet fired horizontally exerts a greater force than the air pushing up on the bullet tumbling through the air vertically.

    bernoulli's equation does not account for the friction due to air, so it would not work.
     
    Last edited: Mar 10, 2008
  13. Mar 11, 2008 #12
    And you also have to realize that due to recoil, the bullet will have a slightly higher trajectory.
     
  14. Mar 11, 2008 #13

    russ_watters

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    Actually, Mythbusters did this very experiment and found that bullets do, indeed, fall on their sides.

    That said, a bullet fired from a gun would stay spin-stabilized for a pretty long time.
     
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