Blog-stalk Confessions: Science quiz...
Wed Jul 14, 2010 8:48:57 pm

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So this puzzler came up in my facebook feed yesterday.

You carefully weigh an enclosed jar containing flies that are asleep on the bottom. Then you shake the jar to wake them up. While they are flying you weigh the jar again. Does the jar weigh less when the insects are flying?

Now my initial instinct was that it would weigh less. Certainly the mass would stay the same, but the scale would obviously not be able to measure the flies as they were in flight...

But the puzzle book disagrees. They say...

The weight of the jar remains the same. While flying, the insects create downward air currents that are equal in force to their weight. Thus, regardless of whether they are motionless or flying, the insects push down with the same force.

Now all the physics make sense, at least the parts I understand. But the sticking point for me is, wouldn't you need to basically ensure that all the insects were creating identical air current force in order for the weight to be the same? Like, if one insect is flying near the top of the jar and one is flying in the middle, then the force generated by each is most likely going to vary just a bit, right? Am I missing something? The air pressure can't sustain its starting force all the way from the fly's wing to the measurable parts of the object, right? Wouldn't it lose a milligram or two of force by the time it reached any part of the jar?

I are confused.