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Thread: Mensa Book

  1. #1 Mensa Book 
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    My wife recently purchased a Mensa book from a charity shop (thrift shop) which was published by British Mensa Limited in 1996. The book is called MENSA presents Logic puzzles.
    I would like peoples opinions on this question and answer:

    Question.
    Carrier Pigeons
    A driver approaches a bridge. He notices that the maximum weight allowed is 20 tons and knows that his empty pantechnicon (removal van ) weighs 20 tons. However, he has a cargo of 200 pigeons which weigh 1lb each. As the pigeons are asleep on perches he stops the vehicle, bangs on the side to waken the birds who start flying around, then safely drives over the bridge.

    Is he correct?

    Answer.
    No. The pigeons remain at 200lbs even whilst flying. Those flying up would reduce the weight, but those flying down would increase the weight, so balancing the total weight.
    end of answer.

    I don't understand this answer. For example, if it was a very tall vehicle and all the pigeons were flying upwards at the same time then does that mean the van weighs less than if they were all flying down at the same time? Surely the answer should have been Yes, the driver is correct. Or am I missing something?


    Richard Smith
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  3. #2  
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    It's probability really. Chances are, half will be flying up, reducing the weight, and the other half will be flying down.

    Realistically, the birds would be flying all over, thus either keeping the weight the same by increasing/decreasing it all at the same time.

    However, technically, the extra 200 pounds wouldn't matter. The bridge builders set the weight limits at what will make the bridge last longest, rather than the total it actually can hold. Hell, he could have been a ton over and the only thing he'd get would be one very large fine.

    EDIT: by the way, you apparently didn't type in the whole question.


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  4. #3  
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    The pigeons would be supplying a 'downdraft' in air equal to their own weight, if the container is 'sealed' then I believe the answer IS correct.
    In fact I question that those 'flying up' present less weight, why?

    because they are having to work harder to climb, and therefore create more downdraft.
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    Quote Originally Posted by Jeremyhfht
    It's probability really. Chances are, half will be flying up, reducing the weight, and the other half will be flying down.

    Realistically, the birds would be flying all over, thus either keeping the weight the same by increasing/decreasing it all at the same time.

    However, technically, the extra 200 pounds wouldn't matter. The bridge builders set the weight limits at what will make the bridge last longest, rather than the total it actually can hold. Hell, he could have been a ton over and the only thing he'd get would be one very large fine.

    EDIT: by the way, you apparently didn't type in the whole question.
    Thanks for that. The question is exactly as shown in the book, why did you think it wasn't complete?
    However, I think the question is stupid as it doesn't make clear that this only works if the vehicle is sealed. If the doors are open front and back and there is air flow through the vehicle then it would be lighter when the pigeons took off - just as it would if the pigeons were on top of the vehicle rather than inside. I also think the answer re pigeons would add less weight when flying upwards as opposed to flying downwards is questionable, to say the least!
    Richard Smith
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  6. #5  
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    The problem with questions of Mensa caliber is that they usually end up lacking description. However, this is to test the people even more. Just use simplistic variables your own mind can think up. Cages aren't closed (the birds would suffocate), etc.
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  7. #6  
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    Here's one From a later mensa (Dec 2006)

    Which number follows this sequence 1921 1315 2021 235 ?

    This was one I did not get, (though I do have the answer) - if you do then well done, I'll change your username to Gigabrain! :wink:
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  8. #7  
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    Quote Originally Posted by Megabrain
    Here's one From a later mensa (Dec 2006)

    Which number follows this sequence 1921 1315 2021 235 ?

    This was one I did not get, (though I do have the answer) - if you do then well done, I'll change your username to Gigabrain! :wink:
    I never understood number sequences, nor found anything on google to help me do so. They aren't prime numbers, as their order doesn't go from greater-lesser-etc. Given my limited mathematical knowledge (highschool so far, I've not taken the initiative to become a mathematician yet), I can't find any relation. However, if you would be so kind as to explain to me how the "sequence of numbers" questions work, I wouldn't have any problem answering in the future.

    Also, my focus point is logic. Not numbers. I will solve logical puzzles much easier (knights and knaves is a favorite. Especially with 5 or more variable people to consider).
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  9. #8  
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    It's nothing at all to do with maths, it is not a mathematical progression, it requires:

    A knowledege of the alphabet and secondly and inspired link/guess/observation.

    A similar puzzle might be:

    'Zelos' is worth 4
    'Tucker' is worth 4
    'Jeremy' is with 6
    'Homouniversalis' is worth 14

    How much is 'megabrain' worth?
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  10. #9  
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    Quote Originally Posted by Megabrain
    It's nothing at all to do with maths, it is not a mathematical progression, it requires:

    A knowledege of the alphabet and secondly and inspired link/guess/observation.

    A similar puzzle might be:

    'Zelos' is worth 4
    'Tucker' is worth 4
    'Jeremy' is with 6
    'Homouniversalis' is worth 14

    How much is 'megabrain' worth?
    I see. So how might one figure out the next sequence? If not the words themselves.
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  11. #10  
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    Zelos is two cyllables each worth 2 points.

    If I got the rest right then Me-ga-brain (3c) should be worth 6.

    I'd have preferred to see 19/21 13/15 20/21 23/5 ?/? 6/18 19/1

    After this the pattern merely repeats.
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  12. #11  
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    Quote Originally Posted by Megabrain
    Zelos is two cyllables each worth 2 points.

    If I got the rest right then Me-ga-brain (3c) should be worth 6.
    No, I mean the numbers originally provided.
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  13. #12  
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    Quote Originally Posted by Megabrain
    Here's one From a later mensa (Dec 2006)

    Which number follows this sequence 1921 1315 2021 235 ?

    This was one I did not get, (though I do have the answer) - if you do then well done, I'll change your username to Gigabrain! :wink:
    1632?
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  14. #13  
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    Quote Originally Posted by Megabrain
    Zelos is two cyllables each worth 2 points.

    If I got the rest right then Me-ga-brain (3c) should be worth 6.

    I'd have preferred to see 19/21 13/15 20/21 23/5 ?/? 6/18 19/1

    After this the pattern merely repeats.
    I edited it before you saw it..
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  15. #14  
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    now you tell me is pent ages on that
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  16. #15  
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    I gave it as it appeared in the book, I have given some clues to make it easier. It is also not based on cylables.
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  17. #16  
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    20/8

    ha, got it
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  18. #17  
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    If that is right
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  19. #18  
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    whoops, that's meant to be 20/18
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  20. #19  
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    20/18 is incorrect, please check your working again...
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  21. #20  
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    no it's 20/8 for thursday
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  22. #21  
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    is it right?
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  23. #22  
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    Quote Originally Posted by Nevyn
    whoops, that's meant to be 20/18
    20/8 is the correct solution.
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  24. #23  
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    blimey that was tough, my head actually hurts, oh well, i like a challenge
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  25. #24  
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    Well then here is another:

    Mary likes 4095 she also likes 8, 24, 399, 224

    Does mary also like 37?
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  26. #25  
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    Quote Originally Posted by Megabrain
    Well then here is another:

    Mary likes 4095 she also likes 8, 24, 399, 224

    Does mary also like 37?
    I give up, it's too tough. I could never do questions like that. Oh well, I'll stick with Gigabrain.
    Come see some of my art work at http://nevyn-pendragon.deviantart.com/
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  27. #26  
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    The previous question (the seven pairs of numbers) refer to the days of the week. the first two letters of each day from sunday are assigned numbers corresponding to their alaphabetical position, Although the word Thursday appears (in one of the answer posts)- it seems even this was not sufficient for one player to cotton on.
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  28. #27  
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    what are you on about, i got it, i put "no it's 20/8 for thursday" look back and you'll see it
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  29. #28  
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    Not talking about you - you got it! I was PM'd by another member.

    Now work on the second one!
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  30. #29  
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    Quote Originally Posted by Megabrain
    Not talking about you - you got it! I was PM'd by another member.

    Now work on the second one!
    oh, ok, yes sir, i'll work on the other one
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  31. #30  
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    Mary does not like 49 or 324 nor does she like 17,22,87 or 107

    EDIT:

    And now for some enigineering logic (by special request)

    I happen to know that the requester has the skills to at least get half way to the solution.

    IF 100 and 10 produces 0
    IF 8 and 12 produces 8
    IF 1010 and 8 and 8 produces 0
    what does 3 and 1 produce ?


    *sigh....*

    Each of the following girls has to work on a project about a famous statesman.
    Yvonne chooses Bismark, Henrietta chooses Stalin, Trudie decides to work on Gandhi, Irene picks Roosevelt, And Virginia chooses Eisenhower.

    Which of the following does Natasha choose?

    Churchill,Mao,Charlemagne, Reagan or Sadat.
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  32. #31 Re: Mensa Book 
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    Quote Originally Posted by deeestuary
    My wife recently purchased a Mensa book from a charity shop (thrift shop) which was published by British Mensa Limited in 1996. The book is called MENSA presents Logic puzzles.
    I would like peoples opinions on this question and answer:

    Question.
    Carrier Pigeons
    A driver approaches a bridge. He notices that the maximum weight allowed is 20 tons and knows that his empty pantechnicon (removal van ) weighs 20 tons. However, he has a cargo of 200 pigeons which weigh 1lb each. As the pigeons are asleep on perches he stops the vehicle, bangs on the side to waken the birds who start flying around, then safely drives over the bridge.

    Is he correct?

    Answer.
    No. The pigeons remain at 200lbs even whilst flying. Those flying up would reduce the weight, but those flying down would increase the weight, so balancing the total weight.
    end of answer.

    I don't understand this answer. For example, if it was a very tall vehicle and all the pigeons were flying upwards at the same time then does that mean the van weighs less than if they were all flying down at the same time? Surely the answer should have been Yes, the driver is correct. Or am I missing something?
    right, to get back to the origial question - and no doubt I'm showing my low IQ here!

    Firstly - the effect of the pigeons flying up or down is a bit of a Red Herring, although I think the answer given in that respect is nonsense.

    But - I understand that the total MASS of the vehicle doesn't change if the pigeons are flying inside it, but that doesn't mean the WEIGHT can't change.
    Let me make a few points:
    1. The pigeons are inside the vehicle sitting on a weighing machine, their total weight is 200lb. The vehicle is also sitting on a weighing machine with the total weight being that of the vehicle plus the 200lb weight of the pigeons. The pigeons take off, the weighing machine inside the vehicle now shows zero lbs but the one the vehicle is on remains the same - why? What is the mechanism for transmitting the pigeons mass to this weighing machine but not to the one inside. Surely if the pigeons are flying and using ENERGY to remain airborn then they become, in effect, weightless - even though their mass will stay the same. Their WEIGHT is transalated into the ENERGY used to keep airborn.
    2. If the pigeons sit on the roof of the vehicle then the total weight of the vehicle is it's weight plus 200lb. The pigeons take off and fly level, lets say, six inches above the roof. The weight of the vehicle is now 200 lb less. If the pigeons are sitting on the floor of the vehicle, inside, then the total weight of the vehicle is again it's weight plus 200lb. If the pigeons again take off and fly level six inches above the floor then the total weight of the vehicle stays the same. Why? What is the mechanism for transmitting this mass to the vehicle in the this second case, and why doesn't it apply to the first case. Could someone please explain!
    3. The pigeons are on the roof of the vehicle and take off, but this time they are tied by strings to the vehicle. They are very strong flyers and using the ENERGY to keep flying in an upward direction the weight of the vehicle not only lightens by the 200lb weight of the pigeons their upward thrust actually lightens the vehicle a lot more, indeed, if they were strong enough they could lift the vehicle off the ground. That is despite the MASS of the vehicle plus pigeons remaining the same! Now, and this is, perhaps the crunch question, if the same pigeons are tied to the floor inside and take off what would happen? Presumably if the vehicle is enclosed they would not be able to get enough lift - too much turbulence - to significantly lighten the vehicle. BUT - I believe that by pulling on the strings, even inside the vehicle, they would still lighten it to some extent. And what if the doors are open to create an air flow, then it would not be any different to them being tied to the roof. So - apart from the lack of airflow, what is the difference - as far a the total weight is concerned - if the pigeons are (a) enclosed in the vehicle (b) in the vehicle with the doors open or (c) on the roof.
    No doubt I am wrong and will look a bit foolish but I still think someone in Mensa has confused MASS and WEIGHT.
    Richard Smith
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  33. #32  
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    Ever stood under a low flying/hovering helicopter? noticed the down draft have we :wink: ?

    THis downdraft spreads out in a cone, the higher the chopper the broader the base of the cone but it is still 'pressing down' on the earth.

    The thrust of a rocket same thing.

    Now consider a slightly deifferent medium yet exactly the same problem.

    Instead of air, lets use water, instead of pigeons, a miniature submarine.
    The sub is on the bottom, the sub rises, is there a difference?

    Now back to the pigeons, what ever they are doing is irrelavant, the fact is they are subject to gravity, they push against the earth to fly, if there is a truck in the way, then they push against the truck.

    If I have not convinced you well then I have not convinced you, it's what engineers call a 'black box problem'.


    And finally the clincher, (ie a black box case).

    Suppose our pigeon was inside a large balloon, which has 14 gms of boyancy, the pigeon weighs 20 gms, and flys around within the balloon, does the balloon start to float? - NO. even though all your logic will tell you it will.
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  34. #33  
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    do i get to become Gigabrain then?
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  35. #34  
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    No, you did not answer the original question, in it's original form, I had to add extra clues, but what about the others I pose?

    There are 5 houses in 5 different colors. In each house lives a man with a different nationality. The 5 owners drink a certain type of beverage, smoke a certain brand of cigar, and keep a certain pet. No owners have the same pet, smoke the same brand of cigar or drink the same beverage.
    The question is: "Who owns the fish?"
    Hints:
     The Brit lives in the red house.
     The Swede keeps dogs as pets.
     The Dane drinks tea.
     The green house is on the left of the white house.
     The green house's owner drinks coffee.
     The person who smokes Pall Mall rears birds.
     The owner of the yellow house smokes Dunhill.
     The man living in the center house drinks milk.
     The Norwegian lives in the first house.
     The man who smokes Blends lives next to the one who keeps cats.
     The man who keeps the horse lives next to the man who smokes Dunhill.
     The owner who smokes Bluemasters drinks beer.
     The German smokes Prince.
     The Norwegian lives next to the blue house.
     The man who smokes Blends has a neighbor who drinks water.
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  36. #35  
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    Quote Originally Posted by Megabrain
    No, you did not answer the original question, in it's original form, I had to add extra clues, but what about the others I pose?

    There are 5 houses in 5 different colors. In each house lives a man with a different nationality. The 5 owners drink a certain type of beverage, smoke a certain brand of cigar, and keep a certain pet. No owners have the same pet, smoke the same brand of cigar or drink the same beverage.
    The question is: "Who owns the fish?"
    Hints:
     The Brit lives in the red house.
     The Swede keeps dogs as pets.
     The Dane drinks tea.
     The green house is on the left of the white house.
     The green house's owner drinks coffee.
     The person who smokes Pall Mall rears birds.
     The owner of the yellow house smokes Dunhill.
     The man living in the center house drinks milk.
     The Norwegian lives in the first house.
     The man who smokes Blends lives next to the one who keeps cats.
     The man who keeps the horse lives next to the man who smokes Dunhill.
     The owner who smokes Bluemasters drinks beer.
     The German smokes Prince.
     The Norwegian lives next to the blue house.
     The man who smokes Blends has a neighbor who drinks water.
    In the fish owner's native language, further encoded in a popular 'language' (so as not to spoil the fun for everyone else);
    0100 0100
    0110 0101
    0111 0101
    0111 0100
    0111 0011
    0110 0011
    0110 1000
    0110 1100
    0110 0001
    0110 1110
    0110 0100

    Cheers,
    william
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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  37. #36  
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    Incidentally, that particular puzzle is popularly attributed to Enistein!.
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  38. #37 Re: Mensa Book 
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    Quote Originally Posted by deeestuary
    Answer.
    No. The pigeons remain at 200lbs even whilst flying. Those flying up would reduce the weight, but those flying down would increase the weight, so balancing the total weight.
    end of answer.

    I don't understand this answer. For example, if it was a very tall vehicle and all the pigeons were flying upwards at the same time then does that mean the van weighs less than if they were all flying down at the same time? Surely the answer should have been Yes, the driver is correct. Or am I missing something?
    I wonder if the author of this solution envisioned that the birds collide with the roof and floor of the truck....

    In that case, a bird bouncing off the roof would tend to lighten the truck while a bird bouncing off the floor would tend to add to the downward force of the truck. And since this is a somewhat random process, as far as the birds not flying all in unison, they would tend to cancel each other's effects and the result would remain a mean value which is roughly equal to their combined rest weight (of course there would be birds bouncing off the walls which wouldn't lighten nor weigh down the truck). But in this case, a bird moving up (and hitting the roof!) would reduce the weight as the author of the riddle's solution states.

    Replace the truck with a container, perhaps a milk jug (the milk molecules being the birds in this analogy). If you cool the jug and weigh it, it will have some value for the weight. Now heat the jug. This will set the molecules in a faster motion and they will collide with the walls of the jug with more force, departing momentum to the jug walls which translates into a pressure. But since this is a random process, there is equal probability that roughly the same number of molecules will hit the top as the bottom (and each side). On average, they will cancel each other's effects and the jug should still weigh the same (but now with a greater internal pressure).

    So much for the collisional scenario.

    If we are to assume that the birds don't collide with the interior of the truck, then I think Megabrain hit the nail on the head. The question then is; would a single bird increase the overall weight of the truck by flying up or down?

    Okay... what if a bird is simply hovering, moving neither up nor down. Flapping his wings just enough to keep him still. There must be a force keeping him from falling - and Newton's 3rd law applies. There must be an equal and opposite force that the bird and the system apply to each other. And since gravity only acts on the bird in one direction (pulling him down), an oppositely directed force acts to keep him in the air. If the truck is a closed container, then that force must be transmitted directly through the air, through the truck, and into the earth and so on. So a hovering bird produces a force on the truck equal to his weight.

    What if the bird is moving up? It takes a greater force by the bird to overcome gravity and move upward than it does for the bird to simply hover. Similarly, it takes less force by the bird to just 'go with the flow' and free fall. In free fall, the force exerted by the bird is zero - a bird in free fall has no weight since 'g' (w=mg) is in effect cancelled by the acceleration of the bird (like how beer bubbles don't rise if the beer is in free fall, or how you would appear to weigh less if standing on a scale in an elevator accelerating downward). So a bird moving downward would not exert as much force on the truck as the hovering bird, yet would still exert a force (unless in free fall). And since the bird moving up has to exert a force greater than that of gravity, that force, like the hovering bird case, would again be transmitted through the air and through the truck and would tend to increase the overall weight.

    Edit: One interesting thing to note is, as opposed to the collisional case where a bird colliding with the roof would tend to lighten the truck, nothing in the non-collisional case would act to lighten the truck. All movement by the birds exert a downward force on the truck greater than or equal to zero (zero being the free fall case). So to answer the question, "Would a single bird increase the overall weight of the truck by flying up or down?" the answer is yes ...
    and more so when moving up.

    Ironically, I don't think it matters much if the truck is 'sealed.' It could simply be a well ventilated cage on the truck bed and I think the same arguments apply. The only difference is that some of the downward force from the bird may be displaced past the sides of the truck and that fraction of the force could make it to the ground without going directly through the truck. But the majority of the downward force would still travel through the truck.


    Another thing to note about this pigeon problem is this subtlety; does it matter if the bird is moving at a constant vertical velocity (i.e., no acceleration) or is it necessary that the bird accelerate? That is, since f=ma, if a=0, and consequently, f=0 but isn't there no force then? Well, the hint to that question lies with the hovering bird. But I'll let this one marinate for a while.

    Sorry I 'pigeon-holed' y'all into reading this.

    Edit: I noticed a few typos in the first draft which may have led to confusion. Should be fixed now.

    Cheers,
    william
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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  39. #38  
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    If it were an open cage then the ratio of velocities comes into play, that is the ratio of the downward draft of the air displaced by the birds wings and the motion of the vehicle. Air flowing through the vehicle may be deflected down yet the truck may have moved on such that the whole downflow is above the 'floor' level as it passes the back of the truck. I 'm not sure of the maximum (unaided) forward velocity of a pigeon though, so I'll leave it as a matter of ratios. IF the truck travels really fast, two things will happen, it will cross the bridge before the road falls away, the birds (if in a cage) will all be 'stuck' to the back wall by air pressure. :wink:

    So as to the birds flying up reducing weight - myth busted. Is it possible the original poster merely mis-copied the article?
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  40. #39  
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    I think if MacGuyver were here, he'd say something like
    "I think the guy should remove the warning sign and just start drivin'."
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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  41. #40  
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    Of course what the truck driver should do is tie all the pigeons feet to the roof, tie their wings fully open, then drive at 100 MPH, the lift from those wings should 'lighten the load a bit.....
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  42. #41  
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    Quote Originally Posted by Megabrain
    Of course what the truck driver should do is tie all the pigeons feet to the roof, tie their wings fully open, then drive at 100 MPH, the lift from those wings should 'lighten the load a bit.....
    ...trucks are usually enclosed. The animals wouldn't feel a thing.
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  43. #42  
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    I said roof, not ceiling, though I guess I can understand...
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  44. #43  
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    Quote Originally Posted by Megabrain
    I said roof, not ceiling, though I guess I can understand...
    ...I thought you meant the roof of the enclosed trailer (roof meaning ceiling), this being a case of bad grammar and word usage..


    However, the birds would die quickly, probably before you reached 60 MPH
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  45. #44  
    Forum Ph.D. william's Avatar
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    To add a riddle I heard about 15 years ago that the pigeon riddle reminded me of; (this one's not too difficult...)

    A man has a chicken, a fox, and some chicken feed. He has to get them across a river by way of ferry, but the ferry owner will only let him take one thing at a time. How does he do it?

    Cheers,
    william
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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  46. #45  
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    Quote Originally Posted by william
    To add a riddle I heard about 15 years ago that the pigeon riddle reminded me of; (this one's not too difficult...)

    A man has a chicken, a fox, and some chicken feed. He has to get them across a river by way of ferry, but the ferry owner will only let him take one thing at a time. How does he do it?

    Cheers,
    william
    Hee...he gives the chicken the feed, and the fox the chicken. Three in one. Hahahahahahaha! Oh that's hilarious!
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  47. #46 Re: Mensa Book 
    Forum Ph.D. william's Avatar
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    Hi Richard (aka deeestuary),
    I just now looked at your website; www.deeestuary.co.uk. Now I know why you like this riddle so much!

    cheers
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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    I'm not sure if any of this freefall and downdraft talk actually makes any sense!
    For starters, surely a bird in freefall is pushing against the air and therefore exerting a downward force.
    But to go back to a previous analogy, if the vehicle was very tall and the birds were high up, surely any movement upwards, downwards, level would be dampened out by the air below them well before it exerted a force on the floor - it would just cause turbulence. As I have asked before: exactly how is the weight of the pigeons in flight transmitted to the weight of the truck? No one has answered that satisfactorily IMHO. And are you really telling me that if the truck was stationary and there was no wind, and that instead of being enclosed the back was a cage with the gaps in the wire, lets say, just small enough to keep the pigeons from escaping - that the effect on the weight of the truck would be the same as if it was enclosed?
    Have a think about this. Megabrain mentioned a submarine analogy. If the submarine was on the sea bed and had 200 pigeons aboard (a bit smelly!), each weighing 1lb. It just so happens that the mass of the submarine keeping it on the floor is 100lb, i.e. it is 100lb heavier than the equivalent volume of water. So if the pigeons were shot out of the torpedo tube the submarine would be 200lb lighter, 100lb lighter than the water and would rise to the surface. But if they stayed in the sub, exactly what effect would the pigeons have by flying around - hitting the ceiling, flying upwards, in freefall, flying level - on the bouyancy of the submarine.
    I think if they all hit the ceiling in unison then that must exert a force upwards, but only for a split second? But otherwise I'm inclined to think it doesn't matter what the pigeons do, the weight stays the same. Why? Nothing to do with downdrafts or freefall but because when in the air the pigeons increase the density of the air (air + pigeons), so increasing the mass of the air. What do you think???
    Richard Smith
    www.deeestuary.co.uk
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  49. #48  
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    Forget all the pigeons hitting the ceiling at the same moment, it is not in the question, but yes it would exert a momentary upwardforce on the truck, but they have to land so the average effect is zero.

    The pigeons are flying around in a closed box, and according to Sir Isaac Newton, (in his first law of motion interia) the box will not move unless an external force is apllied. When flying and not colliding within the box there is no external force.

    Now consider a vertical tube full of air, in this tube is a device which may freely ride up and down yet is in such good contact with the tube that no air can escape past it [a piston]. If you weigh this you will find that although the piston is only supported by the air beneath, it's weight is still measured. You might want to look up 'pnuematic suspension'

    If this tube is on the top of a larger vessel [much larger than the piston] then again the the measured weight will include the piston.

    The pigeons are the same, they are sitting atop a cone shaped block of air.
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    Quote Originally Posted by Megabrain
    Now consider a vertical tube full of air, in this tube is a device which may freely ride up and down yet is in such good contact with the tube that no air can escape past it [a piston]. If you weigh this you will find that although the piston is only supported by the air beneath, it's weight is still measured. You might want to look up 'pnuematic suspension'

    If this tube is on the top of a larger vessel [much larger than the piston] then again the the measured weight will include the piston.

    The pigeons are the same, they are sitting atop a cone shaped block of air.
    This is very difficult to visualise as the pigeons are not in fact sitting atop a cone shaped block of air, the air is whirling all around them. This is different from the piston where, as you point out, the air cannot get past it. However, I am prepared to accept that the net affect of all the air movement is that, when all movment of the air and pigeons is taken in to account, that it is as though the pigeons are, in effect, sitting on a cone of air. This would also explain why if the pigeons are in a cage open to the outside air the cage does in fact weigh less if the pigeons are flying as the 'cone' gets spread well outside the confines of the cage by the turbulence. Am I right in my thinking?
    The pigeons are flying around in a closed box, and according to Sir Isaac Newton, (in his first law of motion interia) the box will not move unless an external force is apllied. When flying and not colliding within the box there is no external force.
    OK, translated in to weight, this means that it doesn't matter what the pigeons are doing - flying up, down, sideways - in unison or otherwise - the weight will not change. If the weight could change, if only for a few seconds, then the submarine would be moving up or down as it's bouyancy changes. But, as Newton laws states, this cannot happen. So whether the pigeons are in freefall or flying upwards, or level - nothing changes. I think we are in agreement with this but not sure if others are!!
    Richard Smith
    www.deeestuary.co.uk
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  51. #50  
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    Yes, you are now more or less there. Yes the cone can be outside if the sides are open.
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  52. #51  
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    Quote Originally Posted by william
    To add a riddle I heard about 15 years ago that the pigeon riddle reminded me of; (this one's not too difficult...)

    A man has a chicken, a fox, and some chicken feed. He has to get them across a river by way of ferry, but the ferry owner will only let him take one thing at a time. How does he do it?

    Cheers,
    william
    First you take the chicken over then drop her off, then go back and pick up the fox, take him over and bring the chicken back, drop off the chicken and pick up the corn, take the corn over, come back and pick up the chicken.
    Eat Dolphin, save the Tuna!!!!
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  53. #52  
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    About the truck/birds question:
    A TV show I watch on occasion, "Mythbusters," tackled this very question. With several experiments, they set out to experimentally verify if the weight of a truck is reduced if the birds are in flight. They also used a radio-control helicopter as a double check.

    Their results...
    the weight remained the same whether the birds were in flight or not.

    It was kind of neat watching that show after taking part in this discussion.

    Cheers,
    william
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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  54. #53  
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    do you really think that the engineer who designed the bridge did not build in a safety factor ? when a sign says a bridge can hold 20 tonnes, surely it can in reality take at least 22 tonnes
    "Reality is that which, when you stop believing in it, doesn't go away." (Philip K. Dick)
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  55. #54  
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    Quote Originally Posted by marnixR
    do you really think that the engineer who designed the bridge did not build in a safety factor ? when a sign says a bridge can hold 20 tonnes, surely it can in reality take at least 22 tonnes

    That was never the question. It's not a question about bridges.

    cheers
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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  56. #55  
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    Quote Originally Posted by Megabrain
    I gave it as it appeared in the book, I have given some clues to make it easier. It is also not based on cylables.
    ive worked out the puzzle !!!!
    I didnt have any paper so I wrote the equation down on my testes. Painful I know but I came to the conclusion that it IS based on silly balls.

    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

    www.leohopkins.com
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