A physics problem:
A bottle with flies inside it is put on a pan of a balance. When the flies rest on the bottom of the bottle, the beam of the balance is level. If the flies buzz in the middle of the bottle, will the beam of the balance keep motionless?
I've found the standard answer is that the balance will keep motionless. But when a fly lifts off or buzzs about in the middle of the bottle, the force that its fluttering wings apply on the bottom is surely greater than its gravity and produces an acceleration. So the balance will sway.
Perhaps in reality the change is negligible and non-macroscopic.
