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Thread: Mind Trap's

  1. #1
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    Mind Trap's

    A famous Magician was carrying three gold coins, each weighing one kilogram. He came to a bridge which posted a sign saying the bridge could hold a maximum of 80 kilograms. The Magician weighs 78 kilograms and the gold weighs three. However, the magician crossed the bridge with all the gold.

    How did he do it?
    To serve man.

  2. #2
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    Re: Mind Trap's

    Come on... this one's too easy.
    He juggled the coins, so that at least one coin was in the air always.
    Trendem

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    Re: Mind Trap's

    Excellent



    Which would be worth more, a pound of $10 pure gold or a half pound of $20 pure gold" or would they be worth the same amount?




    -Trendem = 1
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    Re: Mind Trap's

    1. Trendem, did you come up with that answer yourself? Or have you heard this one before?

    2. From a scientific point of view, I don't know if this would help our magician.
    "I am against religion because it teaches us to be satisfied with not understanding the world" - Richard Dawkins

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    Re: Mind Trap's

    I don't see how it doesn't make sense scientifically. If the weight isn't on the bridge (which it can't be if one is in the air) then he is under or at the weight limit.

    I would think that if we wanted to be exactly scientific about it, then we would concede that he could walk across carrying all the coins. Because despite the sign bridges just don't break if you go 1 Kilogram over the posted weight limit.
    To serve man.

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    Re: Mind Trap's

    Quote Originally Posted by MindTrap028 View Post
    I don't see how it doesn't make sense scientifically. If the weight isn't on the bridge (which it can't be if one is in the air) then he is under or at the weight limit.

    I would think that if we wanted to be exactly scientific about it, then we would concede that he could walk across carrying all the coins. Because despite the sign bridges just don't break if you go 1 Kilogram over the posted weight limit.
    No.

    You have to think about what "weight" means.

    Edit: think about Newton 3.
    Last edited by Allocutus; June 7th, 2009 at 06:04 AM.
    "I am against religion because it teaches us to be satisfied with not understanding the world" - Richard Dawkins

    "If you could rationalize with Religious people there would be no more Religious people" -Gregory House

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    Re: Mind Trap's

    Yea, the total force pushing down on the bridge.

    I look at it from a construction stand point, the bridge is rated for a "live" load.

    Unless a 79Kilogram man.. would break the bridge by simply walking, because he has to exert more pressure than 79Kg, to move.

    I know they have a bunch of really technical debate on it. The "official" answer is "he juggled them". I would personally argue from a construction stand point, and say .. Ignore the sign. Bridges don't fall with "Limit weight + 1"
    To serve man.

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    Re: Mind Trap's

    Quote Originally Posted by MindTrap028 View Post
    Yea, the total force pushing down on the bridge.

    I look at it from a construction stand point, the bridge is rated for a "live" load.

    Unless a 79Kilogram man.. would break the bridge by simply walking, because he has to exert more pressure than 79Kg, to move.

    I know they have a bunch of really technical debate on it. The "official" answer is "he juggled them". I would personally argue from a construction stand point, and say .. Ignore the sign. Bridges don't fall with "Limit weight + 1"
    LOL. Agreed.

    And "Juggled them" won't cut it. When you have 3 pieces of gold in your hands and you throw one of them up, the force of throwing it up causes an equal and opposite reaction (pushing your body down with the same force) --- Newton 3. The other two nuggets are still in your hands. Can't cheat the bridge. Bridges are smarter than us.
    "I am against religion because it teaches us to be satisfied with not understanding the world" - Richard Dawkins

    "If you could rationalize with Religious people there would be no more Religious people" -Gregory House

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    Re: Mind Trap's

    Couldn't he also leave one piece of gold on one side, take two, leave them on the far side, come back and grab the last one?


    Or is that against he rules?

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    Re: Mind Trap's

    Quote Originally Posted by thegreenape View Post
    Couldn't he also leave one piece of gold on one side, take two, leave them on the far side, come back and grab the last one?


    Or is that against he rules?
    Bwahahahahahahaha. Let me rep you.
    "I am against religion because it teaches us to be satisfied with not understanding the world" - Richard Dawkins

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    Re: Mind Trap's

    Quote Originally Posted by MindTrap028 View Post
    Which would be worth more, a pound of $10 pure gold or a half pound of $20 pure gold" or would they be worth the same amount?
    They'd be worth the same surely? Or is there something I'm not getting...
    -=]Eliotitus[=-
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    Re: Mind Trap's

    Quote Originally Posted by MindTrap028 View Post
    Which would be worth more, a pound of $10 pure gold or a half pound of $20 pure gold" or would they be worth the same amount?
    Doesn't it depend on how you define worth?

    If you sell them both, they fetch the same price, $10.

    However, the half pound is worth more per unit of gold. So, techincally the $20 is a more valuble piece of metal because you get the same price for half the material.

    so both could be the answer, no?

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    Re: Mind Trap's

    Quote Originally Posted by greenape
    so both could be the answer, no?
    Nope there is one right answer.

    Quote Originally Posted by ELIOTITUS
    They'd be worth the same surely? Or is there something I'm not getting...
    Nope.

    But process of elimination is going to give the answer soon.
    To serve man.

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    Re: Mind Trap's

    Quote Originally Posted by Allocutus View Post
    LOL. Agreed.

    And "Juggled them" won't cut it. When you have 3 pieces of gold in your hands and you throw one of them up, the force of throwing it up causes an equal and opposite reaction (pushing your body down with the same force) --- Newton 3. The other two nuggets are still in your hands. Can't cheat the bridge. Bridges are smarter than us.
    He could attempt to use a Bernoulli device to keep the coins airborne, or he could supercool the coins to lower their mass (through the concept of linear expansion, or in this case contraction). He could secure the coins to a rock and throw it across the bridge. He could secure each coin to a string and whirl them around his head counter-clockwise, so long as the motion was uniformly perpendicular to the normal force of the bridge.

    Hmm...the density of water is what, about 1000 kg/m^3? He could spit out 0.2 cm^3 of water, and carry the coin across. He could wait until a very strong wind was blowing across the top of the bridge, so that the lift force would help to counter-balance the weight of the coins.

    He could take off his shoes and carry the coins across; he could probably take off his socks, instead.
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    Re: Mind Trap's

    Quote Originally Posted by CliveStaples View Post
    He could attempt to use a Bernoulli device to keep the coins airborne, or he could supercool the coins to lower their mass (through the concept of linear expansion, or in this case contraction).
    This decreases volume, not mass. (I mean, technically speaking to cool it you need to decrease it's energy which decreases it's mass, but the change in the lorentz factor will be quite negligible)


    Quote Originally Posted by Clive
    He could secure the coins to a rock and throw it across the bridge. He could secure each coin to a string and whirl them around his head counter-clockwise, so long as the motion was uniformly perpendicular to the normal force of the bridge.
    Unfortunately, I don't think that would be enough. Just from simple force diagram, you have to have some force which cancels out the gravitational force, and that ultimately has to come from some normal force at the magician's feet.



    You guys are all looking for ways to cheat physics here, and I don't think you're going to find them outside of a very specific way of juggling (keeping only one ball in your hand and applying a very specific amount of force to get a very specific amount of momentum, which may in turn not even work, I'd have to look at the specifics, but you'd need to make certain that you could have precisely two balls in the air and at most one in the hand, otherwise this would never work)

    Quote Originally Posted by Clive
    Hmm...the density of water is what, about 1000 kg/m^3? He could spit out 0.2 cm^3 of water, and carry the coin across. He could wait until a very strong wind was blowing across the top of the bridge, so that the lift force would help to counter-balance the weight of the coins.
    Erhm... Looking a little to hard here, Clive. =P

    Quote Originally Posted by Clive
    He could take off his shoes and carry the coins across; he could probably take off his socks, instead.
    Because my socks have a combined weight of a third of an entire pound.

  16. #16
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    Re: Mind Trap's

    Quote Originally Posted by Allocutus View Post
    1. Trendem, did you come up with that answer yourself? Or have you heard this one before?
    It was the answer that popped into my head two seconds after reading the question.

    Quote Originally Posted by Allocutus View Post
    2. From a scientific point of view, I don't know if this would help our magician.
    Indeed, thirty seconds later, after I had submitted the answer, I wondered whether juggling would make any difference. But then I didn't study physics and I knew MT wouldn't set a question requiring knowledge of physics.
    Trendem

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    Re: Mind Trap's

    My thoughts, from a year of high school physics:

    Juggling doesn't work. There's no what to cheat the laws of physics: considered as a whole, the (closed) system of the magician and the coins weighs 81 pounds, and the bridge only supports 80. My explanation as to why juggling specifically fails: you need to apply an upward force equal to or exceeding the coin's weight to keep the coin in the air. By Newton's third law, the coin applies an equal downward force on you, and you're screwed. Intuitively, bracing yourself for the push upwards puts just enough strain on the bridge that it breaks.
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    Re: Mind Trap's

    Quote Originally Posted by GoldPhoenix View Post
    This decreases volume, not mass. (I mean, technically speaking to cool it you need to decrease it's energy which decreases it's mass, but the change in the lorentz factor will be quite negligible)
    True.

    Quote Originally Posted by GoldPhoenix
    Unfortunately, I don't think that would be enough. Just from simple force diagram, you have to have some force which cancels out the gravitational force, and that ultimately has to come from some normal force at the magician's feet.
    Perhaps some sort of superlight aerodynamic foil, then, to generate lift? Shaped string, perhaps?

    You guys are all looking for ways to cheat physics here, and I don't think you're going to find them outside of a very specific way of juggling (keeping only one ball in your hand and applying a very specific amount of force to get a very specific amount of momentum, which may in turn not even work, I'd have to look at the specifics, but you'd need to make certain that you could have precisely two balls in the air and at most one in the hand, otherwise this would never work)
    So the coins each weigh 9.8N. In order for the downward force to not exceed that, you'd need a<1.0, right? With such a small velocity, you'd have very little air time.

    Because my socks have a combined weight of a third of an entire pound.
    He could have metal socks. Maybe he's MegaMan. Does the problem say he isn't MegaMan?
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  19. #19
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    Re: Mind Trap's

    Quote Originally Posted by CliveStaples View Post
    So the coins each weigh 9.8N. In order for the downward force to not exceed that, you'd need a<1.0, right? With such a small velocity, you'd have very little air time.
    In fact, you'd have 0 air time. To get an initial upward velocity, you need a net upward force i.e. a force greater than 1 lb. Which doesn't work.
    Freedom is you choosing for yourself. Law is the government choosing for you. The two are opposites.

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  20. #20
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    Re: Mind Trap's

    Quote Originally Posted by Castle View Post
    In fact, you'd have 0 air time. To get an initial upward velocity, you need a net upward force i.e. a force greater than 1 lb. Which doesn't work.
    9.8N != 1 lb.

    He does have a bit of wiggle room--2kg worth, or 19.6N. But he couldn't even juggle one coin; the downward force exerted on his hand--mg+F(t) (the force of the throw)--cannot exceed 19.6N, which means F(t)=19.6-g=9.8N, which wouldn't overcome gravity.

    Although technically the gravitational force of the earth would be (extremely) fractionally lower if he was juggling them with his arms fully extended above his head.
    If I am capable of grasping God objectively, I do not believe, but precisely because I cannot do this I must believe. - Soren Kierkegaard
    **** you, I won't do what you tell me

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