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Bullet velocity

  1. Jan 20, 2007 #1
    Hey guys, I just discovered this forum and have a question for yall. I am by no means an expert in physics. I've taken general physics I and II, but my physics knowledge stops there.

    Hate to do this, but I've got to ask a question pertaining to a Mythbusters episode that I just saw. They were testing whether or not a bullet fired directly upward would be lethal when it reached the ground. They concluded from their experimental results that the bullet would not be lethal when it hit the ground.

    From my limited knowledge of basic physics, I would think that the bullet would hit the ground with the same velocity and therefore the same kinetic energy and momentum as when it left the barrel of the gun. Here is my reasoning for this: The bullet leaves the barrel traveling at a certain velocity, and gravity slows it down at a constant rate of 9.8m/s/s. The bullet's kinetic energy is completely converted to potential energy when it reaches its maximum height. The potential energy of the bullet is then converted to kinetic energy as the bullet falls back to earth. So, when the bullet reaches the ground it should have the same kinetic energy, and thus momentum, and when it left the barrel of the gun. I would think that the bullet would be just as lethal as when it was originally fired.

    Sorry for the long windedness, I'm just trying to put my exact thoughts on the screen. Am I leaving something out, or do I have the right idea about this? Thanks alot guys!
  2. jcsd
  3. Jan 20, 2007 #2
    hi jones, your reasoning is correct, but you left something very important out of your model .... air.

  4. Jan 20, 2007 #3
    Thanks allday. So I guess the terminal velocity of the falling bullet (due to air resistance) is less than the initial velocity? I was thinking that its terminal velocity when falling would have to be greater than the initial velocity. So when a bullet is just fired horizontally, does air resistance slow the bullet down progressively to its terminal velocity? (neglecting the fact that gravity would bring it to the ground before it decelerated to its terminal velocity)
  5. Jan 20, 2007 #4


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    Here is a discussion from a while back:
    Original thread found here:
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