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Fireflies: Newsletter Challenge (07/26/05)

Posted July 26, 2005 7:00 AM

The question as it appears in the 07/26 edition of Specs & Techs from GlobalSpec:

While on vacation at the lake house, you and the kids collect fireflies in a jar one evening. After some time, the "specimens" settle down on the bottom of the jar. (Maybe not enough holes in the jar cover?) Jokingly, your daughter remarks, "I bet this jar would be lighter and easier to carry if the fireflies were flying around like they were earlier." Is she right?

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#1

It would be lighter

07/26/2005 12:27 PM

If the flies are flying, then they won't have a weight impact on the jar.

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Guru
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#6
In reply to #1

Re:It would be lighter

07/27/2005 11:26 AM

This was my, and a colleague's, first thought on the matter.

My second thought covered the downforce argument written elsewhere.

My third thought thought was that we should consider whether a pressure vessel containing a vacuum (if that's not an oxymoron - can you "contain" nothing?) is lighter than the same same vessel containing air at atmospheric pressure and is it lighter in both this conditions than when it contains air at a pressure? Surely that's a yes.

I think it would be lighter as the free body diagrams when the flies are in flight would not interact...it is the uplift provided by the flies' wings that counteracts gravity during flight, rather than the bottom of the glass/string/daughter assembly that produces the reaction to the weight when the flies are at rest.

As to whether daughter would notice this difference...how big ARE fireflies anyway?!!

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#7
In reply to #6

Re:It would be lighter

07/27/2005 3:36 PM

how did you factor air into your free body diagram? I think we can both agree that air that is pressurized will cause force. My argument is that the lift given to the flys is at the cost of creating a downdraft, moving air that will come to rest only after it has imparted its energy to the bottom of the jar, or to other air molecules that will do the same. Gravity can not be fooled, there is always a cost for staying 'at rest' on earth.

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#9
In reply to #7

Re:It would be lighter

07/28/2005 3:13 AM

The (static) weight of the air would be the same in both cases. It wasn't the force produced by pressurised air that I was whittering about - it was the increased mass...which logically there must be, but do we notice it? Sorry, it was probably a bit off topic.

I'm not convinced that the downdraft effect (like the weight of the flies) would be big enough to be discernible by darling daughter.

So I'm going to stay sitting on the fence - which I know will feel my weight!

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#2

NO, sum of forces says no.

07/26/2005 1:33 PM

Here on earth, we obey gravity. In the jar, the fly is either laying on the bottom, resting its body on the glass, imparting force to it in response to gravity as it is staying still. If it was flying, it would be imparting the same normal (against-gravity) force to the air, causing the downdraft of air to impart force to the glass. All in all, it causes the same amount of force justifiable as a closed system at rest against gravity. I do not know the specifics of a firefly's aerodynamics, but in general, lift is caused by differentials in pressure and differentials in pressure cause drafts and pressure gradients that would then cause the sum of the fly's weight to be imparted to the glass container.

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#3

Maybe lighter, maybe heavier, until equilibrium...

07/27/2005 12:00 AM

Okay, so this may stretch the context a little, but assuming there are holes, and a little difference in humidity exists between the outside and the inside of the jar - I believe the fireflies being in-flight would create extra turbulence, and until quilibrium, that extra turbulence would temprarily unbalance the zero-sum. They would either raise or lower the weight by raising the rate of air flow through the holes until equilibrium in humidity is acheived. Of course, this would be very hard to measure. Do fireflys sweat during a workout?

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#10
In reply to #3

Re:Maybe lighter, maybe heavier, until equilibrium

07/28/2005 3:22 AM

Do fireflys sweat during a workout? asked WhiteHorse

Well, conversion of complex carbohydrates to energy leaves you with CO2 and water.

Insects respire through spiracles (sp?) in their exoskeleton, so just as we discharge some of the water produced in our bodies by exhalation, so must insects. As far as I know, the only other way they could lose moisture is through stools.

So while they don't sweat with the same mechanism as we do, or for the same reason, the effect of increased water production should be seen (I think!)

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#4

In principle, yes

07/27/2005 3:06 AM

If fireflies are flyng, energy (from muscular apparatus) is generated and globally transformed into heat, which dissipates through the walls of the jar. Lesser energy means lesser mass. So the jar is slightly lighter

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#5
In reply to #4

Re:In principle, yes

07/27/2005 10:32 AM

How are you figuring that lesser energy means lesser mass? In the einstenian sense of E=MC^2? I'm not sure you can equate the presence of energy, the conversion factor to mass, and attribute that to a sensed change of weight to a small child. If you are assuming that spent fuel that remains in the container weighs less than stored fuel (hydrocarbon strains -> water and carbon dioxide -escaping gasses from increased volume) then that's possible, assuming only that the volume of the resulting water and carbon dioxide is greater than the equivalent stoiciometric (sp?) quanitity of oxygen. In general, that would be correct, but the effects of such minute heating and volume generation from flys flying would be nearly negligable considering the relatively chaotic microscopic world of air currents, temperatures, and pressures.

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#8
In reply to #5

Re:In principle, yes

07/28/2005 2:40 AM

I means EXACTLY as asserted by the celebrated Special (and after, General) Relativity formula. The effect will be negligible, but IN PRINCIPLE it must be present. In similar way, if you (as your option) suppose valid the newtonian mechanics: when an apple is falling to the ground, then the center of mass of the body "Earth" MUST advance towards the apple. When a physical sistem is by hypothesis supposed, any consequence (as smallest as it is) is forced to be accepted.

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#11

And the Answer is....

08/02/2005 1:21 PM

As written in the 8/02 issue of Specs & Techs from GlobalSpec:

In a word — no. Many people deduce that because the fireflies are flying, and not "resting" on the bottom of the jar, then they're not putting any weight or downward force on the jar. This is not the case. It becomes clear when you look at it in the following way: when flying, the fireflies accelerate the air downward in order to stay aloft. This is Newton's Third Law: for every action there is an equal and opposite reaction. In this case a firefly forces air downward in order to hold itself up, like an airplane or a helicopter does. So far so good. But what happens to the air? Well, it flows downward until it encounters the bottom of the jar, which of course stops its downward motion. What did it take to do that? An upward force on the air. Work out the forces via equations and you see that the forces are equal: the force generated by the fireflies to hold themselves aloft is imparted to the air which in turn is imparted to the bottom of the jar. The jar's weight doesn't change. (For if it did, then lots of fireflies in a very light-weight jar could fly away and bring the jar with them!)

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Anonymous Poster (1); Camillo (2); Chris Leonard (1); English Rose (3); halcyon_m (3); WhiteHorse (1)

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