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A box of bees

  1. Nov 23, 2008 #1
    A coworker and I were passing time by asking stump the idiot questions. He asked the following question.

    "If you had a box that weighs five pounds empty and placed five pounds of bees inside the box and then shook the box to make all of the bees take flight within the box what would the box weigh when placed upon a scale" (Assume all of the bees remain in flight during weighing)?

    I said five pounds because the bees would be aloft and not contribute to the weight of the box.

    He said ten pounds because the force of the wind against the bottom of the box would equal the weight of the bees.

    What say you?
  2. jcsd
  3. Nov 23, 2008 #2
  4. Nov 23, 2008 #3


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    The downforce applied to the air by the bees eventually is transmitted as downforce to the surface of the earth (think of it as conservation of momentum of the air accelerated downwards by the bees). If the box is sealed, then all of the downforce is transmitted to the bottom of the box. If the box is not sealed, then some of the downforce is transmitted to the air outside of the box, which then transmits the downforce to the surface of the earth.

    The way air exerts a force inside a sealed container is via a pressure differential that decreases with altitude within the container, higher at the bottom, lower at the top, and the net downforce on the container from this pressure differential is exactly equal to the weight of the air, plus the weight of any objects supported by the air, either from downforce applied to the air (flight) or buoyancy (lighter than air balloon).
    Last edited: Nov 23, 2008
  5. Nov 24, 2008 #4
    You can "stump the idiot" with different looks at your example:

    Here are four more ways to foll your friend:

    Einstein said that mass, energy and pressure contribute to gravitational attraction. Hence I would say it weights just a touch more with bees rather than air pressure alone....but it would be too small to measure, so 10 lbs is a good classical answer.

    Also the bees will warm the air contributing energy and hence to mass and hence to "weight".

    Also: Define "empty"
    Is this the weight of the box plus the air inside?? (likely that's what you were thinking)

    How much would it weigh if you evacuated the air?
  6. Nov 24, 2008 #5


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    Yep. 10 pounds. Classic riddle, same as "pidgeons in a truck" riddle.

    Uh, hang on. Rethink that one. Where did those bees get that energy from? They brought it into the box with them. They're not contributing so much as they're converting. In a closed container, the weight will not change by conversion of heat.
    Last edited: Nov 24, 2008
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