How does a fly fly in an elevator?

17 June 2012

Question

Why is it that a fly caught in an elevator (or lift) doesn't hit the floor when it goes up (or, conversely, hit the ceiling if the lift goes down)?

Answer

Dave - You have to think about the forces on a fly. It's got two big forces on it. There is gravity pulling it down and then there's aerodynamic forces pulling it up. If you effectively let the lift start dropping then effectively, gravity is reduced because the lift is accelerating downwards so there's less force upwards on the floor on everybody, and essentially, the lift is moving down around the fly. Now, to start with, the fly has got quite a lot of inertia and it's just going to sit there, essentially staying still when the lift is going to move around it. But a fly has got all sorts of complicated algorithms going on in its head to try and keep it where it wants to be. And I would've thought what goes on is that the fly notices it is higher than it wants to be so it stops flapping its wings as hard, so it will tend to drop down within the lift. And so, it stays where it wants to be essentially just because it flies to where it wants to be.

Chris - So if the fly weren't compensating, which is your answer, it probably would go visibly upwards or downwards according to which direction the lift is moving in?

Dave - Yes, and probably, if the lift suddenly jolted downwards, then the fly would stay still and the lift would move around it and then certainly move upwards.

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