<p>imagine a closed jar with a fly sitting inside. we measure the jar’s mass and weight. now imagine the fly taking off to suspend in the air without touching the jar. does the mass change? how about weight? what if the jar was open?</p>
<p>no…no…maybe.</p>
<p>No mass change in any situation… if there were a mass change, where would it go? Pure energy? Congrats - you just annihilated your home and said glass jar. </p>
<p>As we all know, there’s a difference between mass and weight (i.e. weight can change, mass doesn’t). The weight of the jar, on a super-sensitive scale, may change as the fly takes off (initially heavier as the fly pushes down on the bottom, then lighter as the fly is in the air). Weight = force exerted downwards. Imagine standing on a scale, then jumping up and down. Initially, you would be, say, 10 lbs. Then, when you jump, you are pushing against the scale - so maybe 20 lbs. Then the scale would read “0” as you are in air; when you land, it would read more than 10 lbs. Got it? Now, as for the aloft fly - does your weight change when a hummingbird flies over your head? </p>
<p>Now, the only way that there would be a difference between the weight of a jar with and without lid is if there were a pressure change. Unless you’re going to posit that, I say no change with & without lid. (I guess if the fly flies in such a way as to create air currents that wouldn’t be there with a lid off, you would have a weight differential.)</p>
<p>As for the mass of the system… at any time, it is the sum of:
-mass of the jar
-mass of the air
-mass of the fly</p>
<p>None of these are changing at any time. I’m assuming constant pressure; even when the fly is aloft, it’s not suddenly lighter (much in the way that a 747 doesn’t lose mass when it takes off).</p>
<p>Anyone else?</p>
<p>Actually, if you take the lid off you can increase the mass and weight of the jar/air/fly system (a little). The fly’s efforts could compress the air under the fly (keeping the fly aloft), while the pressure above the fly equalizes with ambient due to the fact that it’s not a closed system. The result is slightly more air in the jar, added from outside. Compressed air has more mass and weight than lower pressure air; thus; the jar/air/fly system slightly more mass and weight, due to a net increase in dense air in the jar. When the top’s on the jar that doesn’t work. The air pressure above the fly has to be slightly lower due to the conservation of mass in the closed jar system. The fly is kept aloft by a combination of greater air density below and lower air density above, with no net change. With the top off that isn’t necessarily true.</p>
<p>Isn’t that what Albert asked? If you asked me, a male, I’d still be wrong; But wouldn’t care.</p>
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<p>I’m not entirely sure – you may see a good deal of pressure equalization through wingtip vortices as well.</p>
<p>Another interesting phenomena is a jar full of water. Lets say you place some kind of mass in the water that floats. Does the weight of the system increase? I’m thinking since water will displace the objects weight to a certain extent (similar to your weight on the moon) that the system’s weight will increase by the objects “new” weight so to say in the water?</p>
<p>No mass or weight change if something is floating. Obviously, mass doesn’t change, but weight doesn’t, either - imagine freezing some water in the jar.</p>