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Treebeard's Stumper Answer
11 December 1998

Jar of Flies

The conservation of mass is the idea that the mass of a closed system, like a sealed jar or maybe the Universe itself, should stay the same whatever happens inside. Of course I can bounce on a scale and change the reading for a moment. Suppose we have a sealed jar of flies. We chill it in the refrigerator and weigh the jar with all the flies resting on the bottom. Then we warm it up and weigh it again with all the flies in the air. Will the jar weigh more, less, or the same? What if we use a wire screen cage instead of a jar?

Jar of Flies image Alice in Chains
Jar of Flies
Columbia Records
Jan 1994

When flies in a sealed jar take off or land, there will be a slight change in the weight of the jar because of dynamic effects, like bouncing on a scale. But if the flies just fly around inside, the jar will weigh the same as if they sat on the bottom. The flies are supported by the air beneath them. Their weight is carried to the jar by the air currents generated by their beating wings. With a screen cage, some of this increased air pressure would pass to the air outside The cage would be lighter, but no longer a closed system.

Notes:

There's an old story that a farmer with a truckload of chickens is stopped by a country bridge, beating the side of his truck with a stick. When asked what he's doing, he explains that the load is too heavy for the bridge, so he's getting the birds in the air to make it lighter.

The weight of the closed jar depends only on its mass, and that does not change when the flies are in the air. But the weight of an open screen cage will change. I guess that's why we don't feel an enormous weight whenever an airplane flies over. Is any increased pressure measurable?

There's another stumper here. If the weight of a fly is carried through the air to the bottom of the jar, isn't it also passed back up to the jar lid and then back down onto the fly? But friction slows down the air. Without friction, the fly couldn't fly, just as we couldn't walk. Putting one leg in front of another would not push us forward without friction to help us push back against the ground.

Dunn Middle School math teacher Susan wonders why we hit the dashboard when the driver slams on the brakes, but flies hovering in the car seem to stay in the same place. (f = ma?)

Insect and bird flight is complicated, though we take it for granted. Despite rumors to the contrary, bumblebees can fly. If an insect or bird just flapped its wings up and down, each downward stroke would lift it and each upward stroke would sink it! I'll save that for a future stumper.

as a representative
of the insect world
i have often wondered
on what man bases his claims
to superiority
everything he knows he has had
to learn
whereas we insects are born
knowing everything we need to know

- don marquis, the lives and times of archy and mehitabel

There's much to be learned from observing how nature really works. A fly's wings must serve the dual roles of propeller and wing, like rotor blades on a helicopter. Flies beat their wings as fast as 200 beats per second! A charming book with some discussion of these matters (and science in general) is Vincent G. Dethier's To Know a Fly (Holden-Day, 1962).

Dunn Middle School students have the holiday assignment to seal something in a jar, and weigh it before and after vacation to test the principle of conservation of mass. I encouraged the kids to be weird and disgusting, and they came through! We let the science building ants test the integrity of the containers for a few days before vacation started.

I expect some change. The lesson is that science is not as empirical as sometimes made out. We don't give up principles easily! In science (and politics), we keep our principles and dismiss contrary evidence because "something happened." Maybe there's condensation on the outside of the glass so it's not really a closed system? I'll report our results after vacation.

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Copyright © 1998 by Marc Kummel / mkummel@rain.org