Fly in a bottle.......

[osprey]

If you weigh an enclosed bottle with a fly at rest in it will it weigh the same when the fly is flying around? If it does, why is this the case?

[Loosehead]

No

[chakka35]

Yes.
When the fly is flying its weight is borne by the air in the bottle. In turn that weight is borne by the base of the bottle. So it makes no difference whether the fly's weight is borne directly by the bottle when the fly is at rest or indirectly via the column of air under the fly.
There is an old puzzle about a man about to drive a van full of pigeons across a bridge, the toll being based on the weight of the van. Would he pay less if he banged on the side of the van to set the pigeons flying? The answer is No for the above reason.

[newtron]

I agree with Chakka35's explanation if the bottle is closed, but what if the bottle is open. For example, if you weigh a drinking glass and then put your finger inside the glass without touching it, the weight of the glass does not change.

[john91]

Thats because the weight of your finger is supported by the earth via your feet or your bottom, not by the glass and air.

[Peter Pedant]

Erm why is this different from

If you have an open bottle or bowl on your scales and drop a ball into the bowl, if your explanations are correct,
then the scales should change immediately and not when the ball hits the bottom of the bottle

PP

[Physics girl]

depends on how easily the air can get out from under the ball? If the size of the ball is negligible compared to the size of the opening then no change until the ball hits the bottom. I reckon.

[jareh88]

to peter.
that wouldnt be the case because the bowl wouldnt be closed off. If i understand what you mean, say you put an empty bottle onto a weighing scale and dropped a bean into it, the weight wouldnt change until the lid is put on due to the theory chakka explained. Is that right?