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This question is very similar to this one or even this one except some minor differences.

  1. The box is much larger than the bee.

  2. The box has no top cover.

  3. The bee is flying still in the middle of the box.

  4. The scale has an infinite precision.

  5. The scale is lying on the ground floor.

Does the scale weigh the bee whether it is inside or outside the box? Does it change anything if the box has a top cover or not?

|                 |
|                 |
|                 |
|                 |
|         (flying |
|       o  bee)   |     (air)
|                 |
|                 |
|                 |
'-----------------'
    | (scale) |
----'---------'--------- (ground)
Community
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nowox
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    If it is in the middle relative to the height, then some non-negligible part of the air flow will be outside the box. See also this question that I asked a long time ago (it was removed at the time for violating the then much stricter homework rules). – Count Iblis Jun 27 '16 at 19:25
  • As this is not a homework question, I feel safe about this :) I had quite a strong argument with some colleagues and I am looking for the truth. This is not an easy question though – nowox Jun 27 '16 at 20:55
  • This is a "homework-like" question. Please show some effort : what do you think? – sammy gerbil Jun 28 '16 at 00:42
  • If the box were completely sealed and filled with water all the way, no air on top and a very dense person were standing on the bottom, would the scale show a different number if the person started to swim? – Peter R Jun 29 '16 at 18:35
  • Wouldn't it be a violation of Newton's third law for a sealed box with air? You would have a completely sealed system for a closed box and the fly would be influencing the force of gravity by flying in the box. – Peter R Jun 29 '16 at 18:41

2 Answers2

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the bee makes movement of air that causes air pressure, and this pressure is distributed around,

if the box is closed, the amount of lift above because low pressure is equal to the weight below caused by air pressure, so no change on weight

but if the box is open, i think that the air outside the box will flow in and the air below will 'hit' the box, basically, the lift will be lost to the ambient, and the weight will change.

in a nutshell:

with a top cover: no change

open box: the bee makes a little amount of weight, depending on its flying height edit:i'm saying here that not all the bee weight will be measured, in fact, the system will weight less than (box mass + bee mass)

Erick Weil
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  • yes. same answer as "if I blow on a scale will it register weight?" – anna v Jun 28 '16 at 04:13
  • I disagree with the "open top" answer. From a simple Newton 3rd Law argument, unless there is a flow of air, downwards outside the box, then the air will produce a force equal to the weight of the bee at the bottom of the box at steady state (ie no change). – Aron Jun 28 '16 at 05:26
  • @annav Completely different actually, unless you are STANDING on the scale. In which case it is exactly the same. – Aron Jun 28 '16 at 05:27
  • @Aron, unless the box have a way to absorb that up force caused by air flow( pressure change), the force that wins is the downward and some of the bee weight will be transfered to the scale – Erick Weil Jun 29 '16 at 14:29
  • @ErickWeil What are you on about? It's Newton's 3rd Law. We are at steady state (1st Law, zero net forces). The bee, sitting on the box, should exert the same force as the bee hovering (what is this "win" business?). The hovering bee is interacting with ONLY the box (yes, the interaction is VIA the air, but at the end, the air stays the same (steady state, Newton's first, therefore the air has no net forces, we can directly couple the box to the bee)). – Aron Jun 29 '16 at 14:36
  • i do not understand what you mean, but what i'm saying is that the bee flying DO causes a 'downwards flow of air' when the box is open, wich can be measured by the scale, this is what orion called 'ground effect' – Erick Weil Jun 29 '16 at 17:46
  • @ErickWeil That would be assuming that the bee is "flapping" as hard at altitude as inside the box. If that were the case, the bee would be exerting a greater down force, and thus be accelerating upwards, violating my first assumption of steady state (or point 3 of the question). – Aron Jul 05 '16 at 06:19
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In an open box, it depends on how high the bee flies. In aviation, this is called the ground effect - if you're flying low enough, you have extra lift, because you're not only pushing against the air, you're pushing against the ground (the air flow below you dissipates and spreads out before reaching ground, if you're high, but not if you're low). A typical example is a hovercraft which obviously does push down on the ground directly.

In essence, the measured weight in the open box will start diminishing when the bee rises higher and higher, ultimately going down to the weight of the empty box. The characteristic height when the bee is no longer relevant, is probably of the order of magnitude of the size of the bee itself.

orion
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  • I find this answer very misleading. This answer implies the bee is exerting constant "effort". The bee would be pushing harder against the ground, but would be accelerated upwards as a result, violating point 3 of the question. – Aron Jul 05 '16 at 06:21