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We humans have dreamed of flying since time immemorial.
Wouldn't it be great if we had wings that we can take off with just like that?
Birds and flies are clearly better off there.
A fly is also the main actor in the following physics puzzle.
She is sitting on the bottom of a closed glass.
There is a second such jar with identical dimensions - only that there is no insect in it.
We now place the two glasses on a high-precision beam balance.
It leans logically to the side of the glass with the fly, because this glass is a little heavier - see drawing above.
What happens to the scales when the fly takes off from the ground and flies a lap in the glass?
(Note: Both glasses are closed at the top, so the fly cannot leave its glass.)
The balance bar may wobble a little.
But the scale continues to lean to the side of the glass with the fly.
As long as the fly is sitting on the glass bottom, your weight will also press the scales downwards.
As soon as it takes off, it continues to exert pressure on the glass bottom.
However, indirectly via the air, which it pushes down towards the glass floor with its wing beat.
When flying, it compensates for its weight through the wing movement.
It can only stay suspended thanks to the constant pressure of air downwards.
The air then exerts an additional pressure force on the glass base that corresponds to the weight of the fly.
When starting, the downward force is even slightly greater than the weight.
Because the fly has to compensate a little more than just its body weight when ascending.
Even when flying, the force fluctuates minimally, depending on whether the wings are moving up or down.
This exciting physics puzzle is available in different versions.
As described here with a fly in a glass - or with a bird on an airplane.
Many thanks to the reader Kai Thöne who suggested it.
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