Geez you guys, why are you making this so difficult?

Hatty's explaination in post #7 is exactly right.

The AVERAGE weight will remain the same, because he will add MORE than his own weight in airpressure at the bottom of the jar as he is assending, and less as he is desending.

By the same token, as a BB is FALLING through a jar of water, the jar will weight less than when it hits bottom. Now if that BB was actually a miniature rocket, (or had wings like a fly) it would have to exert MORE than it's own weight (in airpressure) on the bottom to rise.

As for Philly's comment that the air would bounce off the top of the jar, you are correct, but what you neglected to think of is that the air would then come down again creating a downdraft which the fly would have to fight. Therefore, he would have to beat his wings faster to remain in a hover.

Bottom line... The jar's AVERAGE weight will remain the same.

Always has, always will.

Anything else would break the laws of physics. (and you CAN'T break those laws)

P.S. A wing does NOT create lift by creating a low pressure area above it. If anyone believes that, I have a bridge in Brooklyn that I'd like you to buy.

If you want to know how a wing flies, forget Bernoulli and look up Newtons 3rd law of motion.