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Riddle Gone WRONG

  1. Feb 23, 2006 #1
    This one may take a while to get to the physics but is a good challenge for a mate with a sharp mind. My friends would like to know the answer and the physics behind it, so any answers would be appreciated.

    There was an OLD man out for a walk one afternoon and he decided to walk across a long wooden bridge that leads to a mine on the other side, before he walkes across the bridge he noticed a sign, which stated “this bridge will collapse if anything heavier that 80kg’s is placed on it”. After reading this the old man resumes his walk over the bridge knowing full well he only weighs 78 kg and is safe from it collapsing. He reaches the other side and goes in the mine, a min. later he walk’s out with 3 gold bars each weighing 1kg. The man then resumes on back to the bridge. Once he reaches the bridge he finds himself with a dilemma. He has to cross the bridge with all 3 bars otherwise by the time he gets back for the last bar he fears someone would of taken it, but if he crosses the bridge with all three bars this would then make his weight more than 80kgs (81kg) and the bridge would collapse. How does he do it?

    *you can not swim
    *you can not fly
    *you can not throw the bars
    *the bars must be in a close vicinity to him at all time (1-2m at the most away from him and he must always be holding that majority of the bars {e.g. 2bars})

    The answer you supposable should come up with is …. “He juggles them”. This is were the dilemma comes in, each time the man throws the bar up in the air is he not pushing down on the bridge with a greater weight than the brick itself (e.g. velocity mass force all play a role) and lets also assume he catches the brick with the other hand, wouldn’t there be a greater force exerted by catching the brick than holding it at stand still because of the gain in velocity from gravity on its fall.

    These are all theoretical but the speed at which he tosses the bricks into the air would have a factor in the force pushing down.

    Try and figure it out and remember “for every action there is a greater or equal reaction”
  2. jcsd
  3. Feb 23, 2006 #2

    Doc Al

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    Staff: Mentor

  4. Feb 23, 2006 #3

    Doc Al

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    What's that? Newton's 4th law? :wink:

    (Assuming you are talking about action/reaction forces ala Newton's 3rd law, then "action" and "reaction" are exactly equal and opposite. These forces are better called "3rd law pairs" than "action/reaction".)
  5. Feb 23, 2006 #4
    Ok, weight is essentially mass * net-g force. The man deccelerates and accelerates the gold bars everytime it is passes. The energy he puts into them goes back as energy he gets back. If the gold bars experience an upward g-force greater than 1 g, then the technique is useless when you have two gold bars in the hand at the same time. If the gold bars experience an upward g-force greater than 2 g's, then it is useless to juggle at all.
    Last edited: Feb 23, 2006
  6. Feb 23, 2006 #5


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    In short, flirt, you are right that juggling wouldn't help because of the force of accelerating/decelerating the gold bars.
  7. Feb 23, 2006 #6


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    What if he threw two up, then caught them before they started falling down, after throwing the last one up, and repeating?
  8. Feb 23, 2006 #7
    He can't thorw them.

    What if he two lays one bar on one part of the birdge then he carries the other one then he lays that one down and goes gets the other one and he keeps doing untill he gets across.
  9. Feb 23, 2006 #8
    What about taking off 1 kg of clothes?

    Or what about removing 1 kg of boards off the bridge?
    Last edited: Feb 23, 2006
  10. Feb 26, 2006 #9
    kmarnas86 you thinking out of that square arnt you, that didnt even cross my mind, well done

    scott1 even if he could do that the bridge would still have 3 kg more weight on it and it would be the same as the guy holding them, therefore break bridge
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