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1/2 each

  1. May 14, 2005 #1
    i forget the name of the puzzle but it is somewhere in the brain teaser.

    there is a famus runner in a race with a tortoise.
    and to win he had to go after the tortoise
    but before that he must travel that half of the distance
    but before that he must travel that half of half of the distance
    but before that he must travel that half of half of half the distance
    and so on
    so he go nearrer each time but he cannot go after the tortoise.

    isn't this is a bit simmilar with constantly accelerting and trying to catch up the light speed.
  2. jcsd
  3. May 14, 2005 #2
    The reason you can't catch light is you cant travel faster than light. In your example given a constant rate of travel for the rabbit and the turtle if the rabbit's velocity is faster than the turtle then he will eventually overtake him with time. Time is the key here sure we could keep going 1/2 way into infinity mathematically. But you can also make a formula that will show when in time the rabbit will overtake the turtle. The only way to stop him from catching the turtle is to stop time and thats not possible.
  4. May 15, 2005 #3


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    It's just a case of how you've describe the progress. The tortoise has a head start, by the time the runner gets to where the tortoise was, the tortoise is a bit further adhead. Seems like the runner can't catch up, until you take into account that the time between each instance of the runner catching up to where the tortoise was gets smaller with each cycle, and the rate of these events (versus time) increases until the runner catches up with the turtle where the time between events is zero, and the rate is infinite.

    A similar process is more common place, a bouncing ball. A very hard ball, like a pool (billard) ball bouncing on a very hard surface (tile) is a good example. The ball loses energy on each bounce. If the ball bounces, say 80%, of it's previous height each time, how long before the ball stops bouncing? Turns out it does stop bouncing in a fixed amount of time, but the frequency approaches inifinity as the time progresses to the point the ball stops bouncing. You can hear this frequency increase up to a point. In a real life situation, the bouncing stops sooner, when the height becomes less than the deformation of the ball and surface.
  5. May 15, 2005 #4
    The runner is achilles :biggrin:
    It was a paradox that baffled Archimedeans(sp?) for centuries. They couldnt solve the problem because they didnt know limits. :tongue:
    Last edited: May 15, 2005
  6. May 18, 2005 #5
    Ryoukomaru, yeah that's what i meant.

    so how about the light speed???
  7. May 18, 2005 #6


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    Dearly Missed

    Don't bring Archimedes into this.
    True, Greek PHILOSOPHERS puzzled over this; I've yet to see any evidence that Archimedes was puzzled by this.
    And, you don't need the concept of limits to understand why the "paradox" is flawed:
    Pick any stick of finite length.
    IDEALLY, you could chop up this stick into infinitely many pieces, and then hand over the bundle of pieces to some poor jerk who are obliged to measure the total length of the pieces.
    What he's faced with, is to sum up infinitely many pieces, but still that sum would be equal to the stick's original length.
    Thus, the "paradox" is based on a false premise.

    This is an argument which would have been easily understood by any intelligent Greek, and I am quite certain that we have only been handed down the puzzlements of mathematical incompetents, not the intelligent Greeks' resolution of the "paradox".
  8. May 18, 2005 #7


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    It doesn't really have anything to do with moving at the speed of light. As others explained, the situations are not analagous and converting the word-problem to an equation will demonstrate that.
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