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Benny
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flies in a jar
« on: Feb 25th, 2008, 7:51pm » |
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A bunch of flies are in a capped jar.You put that jar of flies on a scale.The scale will register the most mass when the flies are:
A. Sitting on the bottom
B. Flying around,nearer on bottom
C. ...weight of the jar is the same in both cases
And please also give a reason why.
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Icarus
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Re: flies in a jar
« Reply #1 on: Feb 25th, 2008, 8:39pm » |
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D. On Jupiter. Because gravity is stronger there!
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JiNbOtAk
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Re: flies in a jar
« Reply #2 on: Feb 25th, 2008, 11:17pm » |
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In light of what SMQ's recent explaination on Newton's 3rd. Law, I'll go with C.
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towr
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Re: flies in a jar
« Reply #3 on: Feb 26th, 2008, 1:13am » |
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They did an experiment in mythbusters with a truck with flying or sitting pigeons. Come to think of it, they also tried with a model helicopter.
In any case, when movement is involved it's unlikely the scales will show a stable weight; also E=m c2, and the flies are using energy flying, moreso than when sitting still.
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Grimbal
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Re: flies in a jar
« Reply #4 on: Feb 26th, 2008, 1:32am » |
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They spend energy, but that energy ends up as heat in the air, so it mostly remains in the jar.
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towr
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Re: flies in a jar
« Reply #5 on: Feb 26th, 2008, 2:56am » |
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on Feb 26th, 2008, 1:32am, Grimbal wrote:| They spend energy, but that energy ends up as heat in the air, so it mostly remains in the jar. |
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True, but that would make the jar expand, and thus more 'buoyant'. So it would exert less force on the scales.
So either way, the weight registered would be less 
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Grimbal
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Re: flies in a jar
« Reply #6 on: Feb 26th, 2008, 3:12am » |
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And when the fly breathes out, there is more air in the jar, the jar expands under the pressure, takes up more volume, becomes more buoyant and that decreases the registered weight. Inversely, when the fly breathes in, the registered weight increases.
Another thing. If the fly holds to the cover of the jar, the distance from Earth's center is larger, this reduces the gravitational pull on the fly, so the weight is lower than if it sits on the ground.
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Random Lack of Squiggily Lines
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Re: flies in a jar
« Reply #7 on: Feb 26th, 2008, 1:33pm » |
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but the gravitational pull to the scale would conter the height difference
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Icarus
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Re: flies in a jar
« Reply #8 on: Feb 26th, 2008, 3:51pm » |
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Of course, if the scale is registering actual mass, like the problem says, it certainly would not change, unless it was considerably more sensitive than currently possible.
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Eigenray
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Re: flies in a jar
« Reply #9 on: Feb 26th, 2008, 4:01pm » |
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More seriously, consider the center of mass of the jar.
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Icarus
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Re: flies in a jar
« Reply #10 on: Feb 26th, 2008, 4:36pm » |
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Serious?? It would take an extremely sensitive scale to notice that difference! Even if you could find one, the dynamic forces from the movement of the flies would overwhelm it.
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Eigenray
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Re: flies in a jar
« Reply #11 on: Feb 26th, 2008, 5:06pm » |
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Ah yes, you're right. I was thinking the mass of the air was negligible compared to that of the fly, but if that were the case it wouldn't be able to fly.
If the fly were a bouncing rubber ball, on the other hand, it would make perfect sense to ignore the air. Or maybe a fly bouncing on a spring, or a fly swinging on a pendulum.
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Icarus
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Re: flies in a jar
« Reply #12 on: Feb 26th, 2008, 5:31pm » |
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Actually, my thought is that the center of mass can't change by more than the height of jar, and unless this is one amazingly big jar, any difference in the gravitational field over this distance is going to be immeasurably small.
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"Pi goes on and on and on ...
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I wonder: Which is larger
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Eigenray
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Re: flies in a jar
« Reply #13 on: Feb 26th, 2008, 7:09pm » |
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Sure, but if the COM has any vertical acceleration, there'll be a change in weight. So if we were to ignore the mass of the air (and jar), then W=m(a+g), where a is the upwards vertical acceleration of the fly. But obviously you can't ignore the air while flying -- it's what makes it go.
But instead of flying, suppose the fly is propelled by reflecting a strong laser (or a stream of cricket balls, if you like) off a mirror on the bottom of the jar. Light goes down, pushing fly up, light reflects off bottom, light absorbed by fly, pushing fly up some more. Then we could easily imagine this taking place in a vacuum. Now the weight of the jar actually changes as the fly moves up and down (but it's a function of acceleration, not height).
How about this one (a little more practical): we have a glass of hot water resting on a (very sensitive) scale, and we place a block of ice into it. How does the scale's reading vary with time? [Assume for simplicity no evaporation and that the block of ice remains a cylinder with base area 1, and height h decreasing linearly over time.]
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Icarus
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Re: flies in a jar
« Reply #14 on: Feb 26th, 2008, 7:34pm » |
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Any acceleration is a short-lived effect. I interpret the question as asking what happens when the flies are hovering, not what happens temporarily when they first take off.
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"Pi goes on and on and on ...
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I wonder: Which is larger
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Eigenray
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Re: flies in a jar
« Reply #15 on: Feb 26th, 2008, 8:18pm » |
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Fair enough. Choice C is true, but I don't think it really answers the given question.
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Benny
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Re: flies in a jar
« Reply #16 on: Mar 4th, 2008, 2:40pm » |
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Here's a different scenario:
A jar filled partially with water has an object floating in it. The jar is open. Now, if the jar is closed and a vacuum pump is used to pump the air out, what will happen to the floating object? will it rise up more or sink or ...? and why?
.............................
If you sucked the air out of the jar, then the density of the water would decrease. Thus the object would be more dense than the water. I conjecture that the object would sink.
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towr
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Re: flies in a jar
« Reply #17 on: Mar 4th, 2008, 2:49pm » |
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It depends on many things, the object might expand faster than the water (the expansion of which is negligible in any case).
And even if it doesn't, it may be much less dense to start with.
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