View Poll Results: The scales will:

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  • Read the same in both scenarios

    7 100.00%
  • Read lower for the fly hovering in the bottle

    0 0%
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Thread: Fly in a bottle theory

  1. #1
    Associate Engineer
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    May 2011
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    South Africa
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    Bang Head Fly in a bottle theory

    Hi All,

    At my company this puzzle has raised its ugly head, and the engineers are almost split 50/50 with fair arguments for both viewpoints. ( I strongly believe my aspect is correct, but I wont bias this thread by mentioning it) Most of you have probably heard this riddle but I have been unable to find a technically conclusive solution to this:

    You have two identical flies, two identical sealed bottles and two identical scales. In both cases, the fly is sealed in the bottle and the bottle is sitting on the scale. However, in scenario 1 the fly is sitting still on the bottom of the bottle and in scenario 2 the fly is hovering in the air in one position in the middle of the bottle. (Assume a fly can hover in one place)...

    With some good reasoning, can anyone explain if:

    A. The scales will both have the same reading, or,
    B. The scale with the fly hovering will indicate a lower reading.

    Thanks!

    Lang

  2. #2
    Administrator Kelly Bramble's Avatar
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    Feb 2011
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    Bold Springs, GA
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    I've heard this question before... I will put forth my theory...

    If the total of the thrust from the fly's wings reacts exclusively on the bottom of the bottle then the scales will likely oscillate heavier and then lighter. This would occur because the fly’s wing action does not produce constant thrust.

    Though, I think on average the scales would read less because not all of the wing thrust would act on the bottom of the container. Some of the thrust be dissipated in the friction of the air fluid and some would act at an angle other than perpendicular to scale. The total of the flight thrust will not be applied to the bottom of the container.

    In conclusion, I hope that you folks are not spending too much of your employer’s time on this…

  3. #3
    Lead Engineer RWOLFEJR's Avatar
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    Mar 2011
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    Rochester Pennsylvania
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    Weight's the same because the bottle is sealed. All forces required to keep the weight of the fly in the air are imparted in the bottle... which equals the weight of the fly. If the bottle were open then there would be tiny little fly puffs of force huffing out the top of the bottle. Then I believe you'd have the oscillation Kelly spoke of.

  4. #4
    Technical Fellow
    Join Date
    Feb 2011
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    Not that a I give a great deal of credulity to a group that still insists on using the term, Centrifical instead of Centrifugal, the Mythbusters cleared this one up with flying pigeons.

    As the others before me have stated, the fly's wings are flat and beat air downwards. The thrust to maintain the fly in the air is equal to the weight of the fly so no change on either scales.

    Dave
    Generally, I will not give you the answer to your question, but I **will** guide you into discovering how to solve this yourself.

  5. #5
    "...which equals the weight of the fly..." - nicely put, RWOLFEJR. Agreed, my mental free-body diagram yields the same result.

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