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Don't the morons** firing machine guns in the air kill more people?

Discussion in 'The OT' started by TBlazer07, Oct 21, 2011.

  1. Getteau

    Getteau Icon

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    Dec 19, 2007
    Houston
    One correction. In your example above, in order for the object to come down at the same speed it goes up, the object has to be falling in a vacuum. Without the existence of a vacuum, the ball or bullet will encounter friction from the air and it will stop accelerating when it hits its terminal velocity (assuming the initial force on the object is enough to get it to a height where it can reach terminal velocity on the way down). That's why these types of problems in physics textbooks always have the assumption that the object is in a vacuum. The only way to go faster than the terminal velocity is if there is a downward force acting on the object. However, once that downward force is removed, the object will slow down to its TV.

    In the bullet example, the bullet will be traveling somewhere between 900 and say 3000 ft./s when it leaves the gun (depending on the type of round). If shot straight up, the bullet will come to a stop when the force of gravity and friction overcome the initial acceleration from the gun powder. At that point, the bullet will stop, start falling back to earth and the only downward force on the bullet will be gravity. Given how high a bullet will go when fired in the air, it should hit TV long before it reaches the ground. Looking at this Wiki link below, it sounds like the TV of a 30.06 bullet would be around 200 MPH. I don’t remember what caliber they used in the MB episode, but it must have been a fairly small handgun caliber to reach such a low TV (I don’t recall what they said the TV was in the episode, but I don’t think it was anywhere near 200 MPH).


    http://en.wikipedia.org/wiki/Terminal_velocity
     
  2. Tom Robertson

    Tom Robertson Lifetime Achiever DBSTalk Club

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    Nov 15, 2005
    This mythbusters episode was actually not too bad.

    Yes, they mentioned that they were talking about straight up. That was a clearly indicated proviso.

    They also talked about bullets that remain ballistic rather than reach terminal velocity.

    Their bullets that landed in the ground from a straight up flight clearly did not penetrate the ground as much as a bullet fired directly at the ground. The bullets had reached terminal velocity.

    Now, I suppose one weakness in their analysis was the assumed tumbling. In a more recent episode, they showed how the bullet can continue spinning even after striking ice.

    So if the bullet shot straight up continued to spin on the way down and thus not tumble, it likely would reach a terminal velocity higher than a tumbling bullet yet also at a rate less than the full velocity of a ballistic bullet.

    Aha! Time for more testing. :)

    Cheers,
    Tom
     
  3. Laxguy

    Laxguy Honi Soit Qui Mal Y Pense.

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    Dec 2, 2010
    Winters,...
    'Zackly.
     
  4. Stewart Vernon

    Stewart Vernon Roving Reporter Staff Member Super Moderator DBSTalk Club

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    Jan 7, 2005
    Kittrell, NC
    As was pointed out by someone else, I should have specified "in a vacuum"... but to explain the reasoning.

    Assuming in a vacuum... the only reason why the ball (thrown up into the air in the example) slows down is the force of gravity... that same force of gravity then (at the apex) begins accelerating the ball towards the earth again... and once it has traveled the same distance down that it originally went up, it will have reached its original speed because the force of gravity will have acted upon it the same distance both directions.

    Again, it is basic physics... so a ball thrown up is slowed down by gravity until it eventually stops... then accelerates again... and as long as it doesn't reach terminal velocity it will be traveling at the same speed once it has fallen back to the point it originally was launched upwards from.

    Now, being that we exist not in a vacuum... air resistance will slow the ball on its way up and will hinder it on the way down... so in practical terms it is probably traveling slightly slower once it reaches that point on the way down... but for the most part the principle still applies as long as you factor in the effect of the air resistance.

    I can't begin to say with any certainty the terminal velocity for any specific thing (or in this case a bullet)... and also would the bullet flip over and be more aerodynamic? OR would it keep its orientation and thus be more apt to have a lower terminal velocity.

    I always forget to remind myself that I'm not talking in a vacuum so thanks for pointing that out... I still wouldn't want to be hit by a falling bullet, but I guess it would be better than being shot by one after all.
     

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