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Thread: Engineering head scratcher

  1. #1
    Supporting Member jdurand's Avatar
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    Engineering head scratcher

    This is from discussions I had WAY back when slide rules were made of stone...

    Setup:

    You have an item and a force scale to weigh said item.

    You will find the items weighs the same in a vacuum as it does in air since the air pressure is uniform around it.

    If the item is a sealed container with a bird (and air) in it, it doesn't matter if the bird is flying in there or sitting on the bottom, the measured weight remains the same.

    Question:

    What if the container isn't airtight? Obviously with the bird resting in the bottom the measured weight is the bird plus the container.

    Startle the bird so it is now flying free in the container. Did the measured weight change? Does it matter how large the vents are and where they are located?

    Go...

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    Supporting Member DIYSwede's Avatar
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    DIYSwede's Tools
    Quote Originally Posted by jdurand View Post
    You will find the items weighs the same in a vacuum as it does in air since the air pressure is uniform around it.
    Blue Monday Nit-picking: -I wouldn't find that, as the air doesn't actually exert equal pressure around it.
    It'll be heavier in vacuum (or even cold and/ or dry air, as compared with hot and/ or humid air...
    The Montgolfier Bros was right about this already in 1782.)

    Air pressure is higher on the lower side of the item than on its upper,
    so the item sort of "floats insufficiently", relative to the massive Earth's gravity working on it.

    Air has (like for instance, water) a buoyant force, as the item actually is "submerged" in air,
    consisting of the weight of the air displaced by the item.

    Ref: https://science.howstuffworks.com/tr...r-balloon6.htm

    Oops- I forgot your question:
    Replace the scale suspended container with a biggie cage of thin wires - chicken net.
    -How big a force would the bird suspending pressure exert on the tiny wire area, do you think?
    Last edited by DIYSwede; 10-14-2019 at 03:37 PM. Reason: forgot initial question

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    Supporting Member Crusty's Avatar
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    Supposedly Charles Lindbergh had a fly in his cockpit during a portion of his trans Atlantic flight and he spent a portion of his time musing whether he should 1) Let the fly land inside somewhere 2) Keep the fly moving in the air inside the cabin or 3) Shoo it outside to improve his aircraft's performance.
    If you can't make it precise make it adjustable.

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    Supporting Member mklotz's Avatar
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    mklotz's Tools
    Quote Originally Posted by jdurand View Post
    This is from discussions I had WAY back when slide rules were made of stone...


    Question:

    What if the container isn't airtight? Obviously with the bird resting in the bottom the measured weight is the bird plus the container.

    Startle the bird so it is now flying free in the container. Did the measured weight change? Does it matter how large the vents are and where they are located?
    Stanford has already worked the problem and done an experiment...

    https://www.newscientist.com/article...k-get-lighter/
    ---
    Regards, Marv


    Home Shop Freeware
    http://www.myvirtualnetwork.com/mklotz

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    Supporting Member jdurand's Avatar
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    jdurand's Tools
    Quote Originally Posted by mklotz View Post
    Stanford has already worked the problem and done an experiment...

    https://www.newscientist.com/article...k-get-lighter/

    Thanks, only 3 decades late for my original discussion.


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