Air is a fluid and so is water in physical terms. Air can be a liquid if the pressure is increased. Actually the term liquid or gas is just a convenient but sometimes inaccurate way to classify fluids. An example: if we measure the flow of a liquid at relative low pressures we don’t consider the compressibility of that fluid. But on the other hand, at similar pressures me must consider the compressibility of the gaseous fluids to be accurate.

The jar with the fly inside will only register the fly’s weight when the fly attaches itself to the jar otherwise the air modules support the fly. I’m not considering the heat rise of the air inside the jar due to the poor little fly beating its wings before it lands on the jar.

Think of yourself standing inside a bucket and the bucket the is on a scale. Look at the scale then step out of the bucket. I’m sure the bucket will way less when you are no longer standing in it. Now step back into the bucket, start flapping your wings until you are no longer resting on the bucket. The scale will no longer register your weight.

Miles.

Yeah thats true but we are talking about a closed environment here. Anything that occurs in a closed environment affects its surroundings. When you think about this question, you have to think on the microscopic level. Its hard to do but to understand why this answer is correct, you have to think SMALL.

Air is not a liguid, it's a gas. Mostly... I say mostly because I don't want to argue about every single impurity (or the water molecules) in there. When air is compressed some of it becomes a liquid, not all of it. The molecular bonds are quite different.

Bruce Lee, "Be like water my friend. Be like water."

flies dont sleep

Assume the flies are big enough and mean enough that they weight enough to mess with and that relative sizes are such that Browian motion and the like is not a factor. Sorta like big hummingbirds with many eyes.

With a closed top jar the scale will weigh the weight of the jar plus whatever component of the pressure due to downwash that the bottom of the far feels due to the flapping of the fly's wings plus (or whatever the correct algebraic sign is) the pressure on the underneath of the jar top due to flapping. However, since the scale will always show a weight of the jar plus the flies (closed system) I would guess that the summation of the pressures inside the top and bottom of the jar plus the measured weight-on-feet of the flies is always approximately equal to the flies' weight.

With the flies at rest and no flapping the pressures are equal and opposite. Total flly weight is weight-on-feet. As the flapping increases to takeoff the bottom pressure increases, the upper pressures decreases, the weight-on-feet of the flies decreases but the total of all of that istuff s always equal to the flies' weight.

Since this is always a closed system the size of the jar, the height of the flies above the bottom of the jar and relative sizes of the jar and flies do not effect the final answer.

However -

With an open top jar the scale will weigh the weight of the jar plus whatever component of the pressure due to downwash that the bottom of the jar feels due to the flapping of the fly's wings. In this case as long as the fly is inside of the jar the bottom of the jar will feel the downwash ( or in other words, the forces on the top of the jar are removed). The weight the scale will measure is probably proportional to anything that affects the downwash component of the flies wings on the bottom of the jar - the height the flies are above the bottom of the jar, the flies position relative to the jar mouth, and the relative sizes of the jar and flies.

Sound reasonable, if so I have some swamp land I would like to sell.............

BB's in water is easy...

You have the initial weight of jar and water = to A.
Drop in BB, as the BB is submerged the weight on the scale increases an amount equal to the same volume of water displaced by the BB. This is = to B.
After the BB is submerged and as it falls the total weight is the same as the new above weight.
After it hits the bottom the weight is increased again by the difference between the BB's weight and the same volume of water. This is = to C..

The final weight is the original weight + BB weight.
Note that B + C = BB weight.

Mike_MC:
Trust me. Air or any gas can be liquefied given sufficient compression and removing the heat of compression. No exceptions that I know of. The only difference between a gas and a liquid it the density of the fluid. The higher the density the more “liquid like” the behavior, lower the density and the fluid behaves “gas-like”. Even water under reduced pressure (aka vacuum) becomes a “gas-like” substance. Our notions of liquid and gas are relative to our environment. If you lived on the Moon or Mars water could only exist as a gas (due to the low ambient pressure) or as ice (due to the low temperatures) which is really just a real dense fluid. Of course this really doesn’t mean anything except to engineers and others interested in the physical properties of substances. We often refer to air, or a gas as a fluid and it makes sense to us and I can appreciate the confusion it generates for others.

The rest of this is making my head hurt. I’m retired and have quit thing on a regular basis. Sometimes I think my IQ has dropped 20 points…

Ben Lanterman:
A closed jar without a fly will weigh n cubic units times the density of the air. Jar with a fly will weigh: (n cubic units X the density of air) less (the volume of the fly times the difference in the density of the air and the weight of the fly). With a sensitive enough measurement you could (?) measure the velocity of the air coming off the fly’s wings and impacting the bottom of the jar, assuming the scale is under the jar. It would appear as a pulsation due to the up and down movement of the wing. But if I were conducting this experiment I’d just wait for the little bugger to tire and settle down and I’d call it a static measurement. The BB and the water (or liquid) is exactly the same. I think we’re in close agreement, right?

Have a good day and let’s release those flies and go flying!
Miles.

Ok, So how many invisible angels can dace on the head of a pin?

Jim

Yes, air can be compressed and get some amount of liquid out of it, but that does not mean they are the same thing. Different porperties and different bonds.

"air" is simple not just air. When compressed the water that's already in the air simple condensates. The water molecules form a "weak" bond to each other that didn't exist before. This "weak" bond is what makes the difference.

Let's re-frame the question.

If all living things in the universe jumped "up" (or stopped flying or walked "east" or whatever as long as all living things did the same thing at the same time) would the universe weigh more than it does in its current state?

I say no, the universe would weigh the same regardless of what the living beings within are doing.

This is the best thread yet on RCU! I love it!

More details needed ..

What are the names of these flies?

-DC

Mike_MC:
Well, what more can I say? Maybe, you’ve heard of liquid nitrogen, or liquid oxygen, just to name a few of the “liquefied gases” that are manufactured by compressing air? Just about all constituents of air, that have a commercial value, can be purchased as a “liquid”. Also, don’t be confused, I’m not at all talking about the moisture (aka water vapor) in the air, I’m only talking about the “gas” components. The water vapor must be removed before the liquefaction of the gas components can start or the water vapor will turn to ice and plug-up the equipment. Well, I’ll just have to let you live in your world while I live in mine.

But if you want more information go to AIR LIQUIDE (a French company) web site and you’ll see lots of liquefied gas products they produce and each come from the air you breath.

Miles.
PS I’m not trying to be contentious or argumentative, I spent a large portion of my 25 years as an engineer in the gas measurement and compression industry sometimes as a consultant and sometime as an in-insult-ant. Just joking about the last.

In principal, the law of energy conservation prevents the universe from altering its energy/mass, ignoring the possible violations introduced by virtual particles, but within a local frame of reference, and assuming that “life” or biological activity isn’t uniform across the universe, of which we have no evidence to date indicating such, I would be hesitant to make a make a firm prediction of what possibilities that could be observed if such actions taken. That being said, I think the chief obstacle, after the initial recruitment, would be the timing.

But on the other hand, the laws of both energy conservation and thermodynamics are essentially mathematical laws—they depend relatively little on the specific details of our universe. Just about the only specific detail that's important is time-translation symmetry: physics doesn't change with time--physics today is the same as it was yesterday and as it will be tomorrow. That observation leads, amazingly enough, to energy conservation: energy/mass cannot be created or destroy; it can only change forms or be transferred between objects. So, getting all biological entities in our universe to move at the same time, in the same direction could lead, in our local frame of reference, to a large mass to energy transfer producing enormous heat putting our global warming issues on the back burner, in a manner of speaking.

Miles.
PS if this makes sense to anyone please keep it to yourself.

Mking: You're arguing about something I didn't say. Please re-read.

You're talking about elements (nitrogen and oxygen) now... air is not an element. We were talking about air, but that's even beside the point. I never said it couldn't be compressed (or other methods) to get some liquid out of it. Everybody knows many elements and compounds can change state. I never said they couldn't. I simply said gas and liquid are NOT the same thing. No matter how you twist it, gas and liquid are NOT the same. They have different bonds and different properties, not just the density. Known facts about one can NOT be assumed the same and applied to the other. i.e. If you condence air into liquid and then freeze it can you still fly throught it?

BTW, "air" can NOT be turned into a liquid. There's no such thing as liquid or solid air. Some of the compounds/elements in air are already a liquid or can be condensed (or whatever method) into it. Some are already solids. Nothing of what you're talking about is made by simply compressing air.

By now I've already put the jar of flys on the pool heater and cooked them because I just can't go on with this.

Please forgive me if I misunderstood you. But AIR (70% n2, 21% o2 + others ELEMENTS) can be liquid. ONE more time: compress the air, remove the heat of compression and you get liquid air of the same mixture that you started with. You only get the purified elements after fractionation. End of discussion. You are welcome to live in your world and I’ll live very well in mine.
Miles.

are we in agreement that no matter what the fly does the jar weighs the same, as in the glass and lid, THAT weighs the same ( hhehe trick question if worded so in the question )
what we're talking is the flys weight at rest, are the forces it puts out in flght = to it's weight. whos got a set of scales?

has anyone ever jumped in a train wondering if the train will move while your in the air and you land somewhere else??? i have and was disapointed to land in the same spot lol to young to know about momentum, but i'm sure now thats the longest leap i've ever made lol

Good point about the train. That is a perfect example of how a closed environment differs from an open one.

ps.. it works better on busses just as they pull away. imagine 20 kids all on the top deck doing that at the same time lol, even more fun when they stop.

I’ll completely agree with your conclusion if you substitute mass for weight.

You received momentum from the train therefore giving you the impression of landing in the same spot. If you disengage yourself from the floor of the train for sufficient period of time you would notice a displacement in your location due to the train’s continued forward motion and your loss of momentum. But if you were to jump while the train is increasing or decreasing its speed then this difference would be much more noticeable. Also consider what happens when the train suddenly changes it speed which is most noticeable while standing: your body continues to move in a direction opposite that of the train’s until your momentum equals the train. If this weren’t true then nothing would happen to you in a car crash.
Hay that’s my story and I’m sticking to it.
Miles.

I'm not sure how you think I was even talking about that to start with. One more time... I said gas and liquid are not the same thing. I give up on what they're made of or how you make them, you win there, but my main point is, they are not the same no matter how they're made or what they're made of. They don't have the same porperties or bonds. When you compress it and remove the heat to make liquid energy in the form of the heat you removed is being release and chemical reactions are taking place. a-la the bonding.

I guess we can keep arguing about different things for along time here if we want.

...and the flys are very well done now if anybody wants some.

I'll jump in here on the gas and liquid thing.

My book says "Elements of Fluid Mechanics" on the front cover. Fluid is used as a describer of things that can be either gas or liquid since the basic mechanics can be handled by similiar math stuff (I hated math stuff, when I got to college it was like a car hitting a brick wall when I discovered that my brain wouldn't handle higher math stuff, a sad day).

I can remember being taught that we could use words such as "gas" and "liquid", so named because they are DIFFERENT states of the same thing. If not why use two different words??

Same THING but different STATES. (THING is a scientific word to cover a lot of STUFF)

The fact that you can change from one state to another is good and means that with respect to things moving in them, the gas and liquids act differently.

In each case there is a size of thing moving to gas or liquid relationship that makes little things in gas behave similarly to little things in liquid (sorta, kinda, with some weasel words, depends on who is looking and at what.)

Did I miss anything here??

Miles - Agree except what would cause the loss of momentum of the jumper in the train that is traveling at constant velocity? Assume that everything is linear, no earth curvature or relativistic effects (cheating yes but handy).

The only forces acting on the jumper while in the air aero forces which are probably at a steady state condition. Gravity is constant down. I can't come up with anything that would give an acceleration to the jumper relative to the train in the longitudinal direction.

Ben

Let's say that the jar sits for a few weeks. Since there wasn't sterile conditions in the jar, what with all the flies and such, that eventually the flies will die and fall to the bottom of the jar, where over time, bacteria will multiply, due to the process of decay, and therefore will increase the mass of the inside of the jar, assuming that they will increase in mass more than the decrease in mass of the fly due to the fact that the bacteria are eating it. Also, bacteria and decay produce gasses, so the pressure inside the jar should increase, thus increasing the mass in the jar. So, regardless of how hard those little guys fly around, eventually they have to croak, and then for awhile anyway, they will weigh more. Sure, that's it...

Thanks Ben. The real problem is Miles' and my deffinition of "same". I'm talking about same (or not) same properties. Miles is talking about chemical makeup... unless I missed something.

BBQ flys are nice and crispy now if anybody wants some.

Ben

The traveler only receives the benefit of the train’s forward movement when he and the train are in physical contact. Consider sitting in a seat on a stationary vehicle when the vehicle starts its forward movement you are pressed back into the seat. Of course the greater the acceleration the more you are “pressed back” into the seat. This pressed back into the seat is the transfer of momentum between the vehicle and you. Once the acceleration has stopped the energy transfer is still taking place while you are in physical contact but is much less than during the acceleration phase.

Breaking this physical contact with the moving vehicle does not stop the transfer of momentum but the transfer is much less because now only the air (being much less dense than the seat) inside the vehicle is transferring the energy and the principal force acting on your body is now gravity. Momentum is now transferred to a downward direction and slowing down your forward momentum at approximately 32 feet per second per second.
Miles.

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.