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17 replies to this topic

#11 IrNinjaBob

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Posted 19 August 2007 - 11:17 AM


but riddles should only have one possible answer.




Who says?



Good point. There can be riddles that have different answers. What I meant was when you have a riddle with only one answer, it should only be possible to get that answer. If you are going to say "no" to an answer that technically would work simply because it wasn't the one you had in mind, then you intended there to be only one answer. To accomplish this, you add the extra information specifying what you can and can't do. Other than that, I thought this was a neat riddle. With that extra information this riddle would have been a lot of fun to work out.
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#12 Martini

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Posted 19 August 2007 - 03:27 PM

yes you're right. but it doesn't matter on the question. i didn't emphasize this to take into consideration. this was based on a simple math.


Sure it matters. Riddari is correct that juggling will not allow the man to cross the bridge and that's what matters. Here is a good discussion on the riddle.
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#13 bonanova

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Posted 22 August 2007 - 11:10 AM

Doing the math and explaining that downward forces are needed to produce upward acceleration, etc. is absolutely correct; even tho the numbers change depending on how, and how high, he throws them. One might be tempted to suggest he could throw them very gently upward ... this approach fails, but more math is needed to prove it.

A general argument states: If the total weight of the juggler and the watermelons is NOT being supported by the bridge [the only supporting structure present] then some or all of them will fall, and juggler will fail to get them across. Thus, the bridge DOES support the entire weight, and juggling is seen not to be a solution.

... unless ... the bridge is so short he can toss them into the air before getting on the bridge -- and catch them after he has finished crossing it.
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The greatest challenge to any thinker is stating the problem in a way that will allow a solution.
- Bertrand Russell

#14 cpotting

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Posted 22 August 2007 - 05:35 PM

Doing the math and explaining that downward forces are needed to produce upward acceleration, etc. is absolutely correct; even tho the numbers change depending on how, and how high, he throws them. One might be tempted to suggest he could throw them very gently upward ... this approach fails, but more math is needed to prove it.


Throwing them "very gently upward" will not work - it does not change the amount of force needed - just the amount of time the watermelons spend in the air. Think of it this way: if he just stands there with a watermelon in his hand, he exerts 80kg down force (79 for himself, 1 for the watermelon). To move the watermelon upward - at any speed - requires additional force be exerted downward, exceeding the 80kg limit and collapsing the bridge.
(In reality, kgs are not a unit of force. The bridge is capable of supporting 80kg x 9.8N/kg = 784 Newtons of force. The man exerts 774.2N and the watermelon exerts 9.8N. We can just stick with kgs for simplicity)

... unless ... the bridge is so short he can toss them into the air before getting on the bridge -- and catch them after he has finished crossing it.



I like that solution. I had thought about horizontal juggling (yes, it exists - I do it myself). Basically, think of regular juggling of three items, but reduce the height. Keep reducing it until the majority of the force used in throwing the ball is along the horizontal plane rather than vertical. At this point you are juggling very quickly, but with very little upward force. Unless the bridge has really bad horizontal stability, this would allow juggling to get the melons across. But to do this with watermelons would be extremely difficult. And he must still walk very slowly, so as not to exert more than 9.8N while lifting his body weight or placing his foot. That would be unlikely.

I wonder if he could bowl the watermelons across and then walk? I guess that depends on the length of the bridge.

I say that he should walk downriver to a bridge that has been properly constructed, get is watermelons across, then call the newspaper to report on the shoddy construction practices, lack of inspections and obvious safety concerns raised by his recent ordeal. Someone should be doing jail-time over this!
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#15 normdeplume

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Posted 22 August 2007 - 05:48 PM

The man should exercise vigorously for about an hour, Boxers seen to find skipping does the trick, once he has lost 2 KG, he will be able to casually walk across proudly holding on to his watermelons (is it just me or does that sound like a euphemism, perhaps he hurt them skipping?)

Or if he was a Catholic, he could just give up religion, he would then be able to get across easily (less Mass)
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#16 sajow4

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Posted 22 August 2007 - 07:24 PM

I think that "IrNinjaBob" is quite intelligent, and quite correct. In many riddles, there are more than one answer (some sarcastic nd unlikely, but nonetheless able to solve the puzzle), and there are some which there is only an absolute one answer. This is because of the information given. If the information given is too broad, like "What can you put in a bucket to make it lighter?" can have multiple answers. If the person posting a question wants merely one answer, than the person should put more information, else he or she will get many responses.

For the puzzle, I think that throwing the watermelons would increase the weight, as "Riddari" had said, and the bridge would crack (as well as the watermelons would most likely break, but that is out of question and unneeded).

My answer is that the man carry one water melon with him, weighing a perfect 80kg, put down the water melon on the other side, than walk back to the other watermelond, weighing 79kg, and then carry that watermelon back to the other side, weighing a perfect 80kg. This answer is plausible, reasonable, and simple, is it not?
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#17 bonanova

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Posted 25 August 2007 - 06:29 AM

... the man carry one water melon with him, weighing a perfect 80kg, put down the water melon on the other side, than walk back to the other watermelond, weighing 79kg, and then carry that watermelon back to the other side, weighing a perfect 80kg. This answer is plausible, reasonable, and simple, is it not?


Plausible, reasonable and simple. Compelling, even.

Oh wait. The problem states:

"This bridge capacity is up to 80 kgs only, if you exceed this limit the bridge will fall".

"up to 80 kg only" [an open interval] does not include 80. Therefore "a perfect 80kg" "exceed[s] this limit," and so "the bridge will fall."
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#18 sajow4

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Posted 26 August 2007 - 02:53 AM

"This bridge capacity is up to 80 kgs only, if you exceed this limit the bridge will fall".

"up to 80 kg only" [an open interval] does not include 80. Therefore "a perfect 80kg" "exceed[s] this limit," and so "the bridge will fall."



You are correct, my mistake.
Seeing this as not an option, pursuing other ideas:
Throwing it - This would work, theoretically, if it does not matter if the watermelon is damaged.
Rolling it - This would have identical effects, only the water melon would not be broken beyond holding, but it may have a few dents.

the bridge is so short he can toss them into the air before getting on the bridge -- and catch them after he has finished crossing it.


Yes, this couldwork as well, as the length of the bridge is unspecified.
Well done, bonanova.
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