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Another bullet question?

  1. Sep 26, 2007 #1
    This is my first physics question (and hopefully not my last)

    I've been wondering about what happens when you fire a gun in a situation with no gravity (or atleast no gravity of the Earth's) but when there is air and frictional forces present. People have tried to convince me that the bullet attains its terminal velocity and then continues in a straight line (infinitely)

    Is this possible ? :confused:
  2. jcsd
  3. Sep 26, 2007 #2


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    Welcome to PF, Cooldood.
    The bullet will cease accelerating as soon as it leaves the muzzle (actually, a brief fraction of a second later due to residual pressure following it), and then indeed carry on in a straight line. The shooter will also be moved backward by the recoil force, which is equal to that carried by the bullet.

    edit: By the time I got around to answering this, I forgot that I hadn't read the entire question. I'm at work, got interrupted part-way through, then finished without seeing the part about 'infinitely'. :redface: It will indeed slow down and eventually stop due to friction and compression forces, but it's path will be a straight line. Sorry, guys.
    Last edited: Sep 26, 2007
  4. Sep 26, 2007 #3
    Not possible. Air friction keeps slowing down the moving body, until it stops.
  5. Sep 26, 2007 #4

    Doc Al

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    Staff: Mentor

    Terminal velocity requires a balance of forces; that's not the case here. As long as air resistance (the only force) continues to act, the bullet will continue to slow down.
  6. Sep 26, 2007 #5


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    No, the term "terminal velocity" refers to the velocity where the force due to gravity (g, m*g, where g is about 9.8 m/s^2) is "balanced out" by the friction due to air (which is proportional to the speed of the bullet).
    Hence, in the absence of gravity (and forces from e.g. from a rocket) there is no such thing as terminal velocity.

    This means that the bullet will have reached its maximum velocity when it leaves the barrel of the gun, then it will just slow down due to friction until it stops.
  7. Sep 27, 2007 #6
    Thanks a lot everyone :)
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