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Pop Quiz 4: Which weighs more?(72 posts)

Pop Quiz 4: Which weighs more?jose_Tex_mex
Oct 16, 2002 3:46 AM
I just thought I would lighten the mood of the board - too much terrorism, snipers, ...

Anyhow, most of you are doing well and it's nearly time for report cards so here is another pop quiz.

You have a van with a ton of bees inside sitting on the floor. Which weighs more - the van when the bees are sitting on the floor or when they are flying inside the van?
Are youEager Beagle
Oct 16, 2002 4:51 AM
weighing the bees collectively, or individually?
re: Pop Quiz 4: Which weighs more?DougSloan
Oct 16, 2002 5:35 AM
The van always weighs the same.

If you are talking about the apparent weight of the van, plus the bees, I'd say more when they are on the floor. When they are flying, they are not exerting any pressure on the floor. That's my guess.

This is all moot, though, as science has proven that bees cannot fly.

Doug
Now Doug...Wayne
Oct 16, 2002 6:34 AM
don't bash us scientists, no scientist would ever claim bees can't fly, since you can easily observe them. I think it was a (bio)engineer who claimed bumblebees shouldn't be able to fly. Perhaps some of his assumptions in his model were wrong, either that, or Bumble Bees are proof that God does intervene daily to defy the physical laws of the universe.
fyiDougSloan
Oct 16, 2002 6:45 AM
It is obvious to any scientist that the bumble bee can fly as experiment proves it. So what is this business about proving bees cannot fly? And who started it?

First let's look at the physics behind the story. If you are asked about flight the first thing you do is to use the equations which describe how much lift an object has. You compare the lift to the weight of the object. If the lift is greater than weight then the thing can fly. Bumble bees are pretty big, weighing almost a gram, and have a wing area of about a square centimetre. Tot up all the figures and you find that it cannot generate enough lift at its typical flying speed of about one meter per second. But that doesn't prove bees cannot fly. It proves that bees with smooth, rigid wings cannot glide. Experiment has proven this too. With the aid of dead bees and a little lacquer it is easy to show that they really cannot glide.

So how do they fly? Actually that turns out to be a very interesting question and one that reveals a lot of physics. Why do bees flap while jumbo jets have fixed wings? It is a question of size and this is revealed in a figure called the Reynolds Number. Osborne Reynolds was a Victorian engineer who was interested in what happens when you place an object in a stream of liquid or gas. The number named after him is a ratio which tells us, for a particular object, how much lift you get compared to how much drag or resistance you get. A low Reynolds number means little lift for a lot of drag and a large Reynolds number means a lot of lift.

The Reynolds Number depends on the size of the wing. Bigger wings give bigger Reynolds numbers. Now if, again, you put in all the numbers you find that bees work at very low Reynolds Numbers (1000 or so for a honeybee, as little as 15 for the aphid-eating chalcid wasp). This means that their flight is very inefficient because as a wing starts to move to create lift the drag holds it back. It is fairly straightforward to show that birds can generate enough lift to fly once they are in motion with air flowing smoothly over their wings, but many of them would have great difficulty taking off. Small insects, according to this model, cannot fly at all. Of course, all this proves is that the model is incomplete.

Some brilliant work by Torkel Weis-Fogh has shown us how small insects do fly and it has led to some rather neat insights into nature's cunning. If you are small and want to fly you have a problem. The Reynolds Number is against you so you cannot glide and flapping is very hard work. A wing is a device which encourages the air to flow over it so that when it leaves the rear wing edge, the air moves downwards. That produces a thrust upwards on the wing. A smoke-filled wind tunnel shows this beautifully with curling eddies of smoke flicking off the wing edges. Unfortunately to make a good eddy takes time. The wing has to move a few times its own length to get things started. This makes it tricky if you are going to flap as the maximum travel of a wing is about its own length and very little lift is generated for most of the stroke. Nature has come up with a number of interesting solutions to this problem of which the "clap-fling" is a good example. When a small bird or insect wants to take off it needs a lot of lift. What it does is bring its wings together above its back so they clap, expelling air from between them. As the wings are separated, air is drawn quickly in to fill the void. The wings are flung apart and lift is generated immediately as the air is already in motion in the correct way. You can hear the clap. The characteristic whirring of a pheasant taking off is caused by its wings clapping. Almost 2000 years ago Virgil recorded in The Aeneid that a rock dove claps its wings as it takes off - a passage he stole from Homer but he added the bit about the clapping.

So in asking how bees fly we find that they are remarkably clever about it. Aircraft can generate enough lift t
fyi - rest of itDougSloan
Oct 16, 2002 6:46 AM
So in asking how bees fly we find that they are remarkably clever about it. Aircraft can generate enough lift that they do not need such tricks, but they do need long runways. Birds get enough lift to fly but for take-off need a boost. Just the poor old bee and about a million different species of winged insect need some extra trickery to stay aloft.

But how did it all start? Where does the story date back to? J.H.Mcmasters states that the story was prevalent in the German technical universities in the 1930's, starting with the students of the aerodynamicist Ludwig Prandtl at Göttingen. The story he tells is that a noted Swiss aerodynamicist, whom he does not name, was talking to a biologist at dinner. The biologist asked about the flight of bees and the Swiss gentleman did a back-of-the-napkin calculation of the kind I described. Assume a rigid, smooth wing and so on. Of course, he found that there was insufficient lift and went away to find out the correct answer. In the meantime the biologist put the word around, presumably to show that nature was greater than engineering, and the media picked it up. The truth, as now, wasn't newsworthy so a correction has never been publicised. The man on the Clapham omnibus, therefore, continues to tell me that science is a load of crock because it once proved that bumble bees cannot fly.
You are being a little harsh.Eager Beagle
Oct 16, 2002 7:10 AM
I am a man, and I used to spend rather a lot of time on the Clapham omnibus, as it ran past my house.

As it spends most of its time entirely stationary in traffic, ones mind tends to wonder.

Mainly, it wonders not if bees can fly, but if the internal combustion engine can be utilized so as to impart forward motion tp vehicles. Any experiment with rapid movement would be almost impossible to visualize...

Actually, the most amazing question is now the judge who immortalized that phrase even knew that there WAS a Clapham omnibus.
The man on the Clapham omnibus...Wayne
Oct 16, 2002 7:12 AM
must be of the same ilk as those that claim human paleoanthropologists can't possibly know what they're talking about because they were duped with Piltdown man! Very interesting stuff, where's that from? I'm reading a book by Vogels (Life's Devices) that explores the ways life "solves" the problems that "physics" impose upon it.
My favorite (paraphrased quote) so far, "Drop a mouse down a 1000 ft mine shaft and it is dazed by the impact but walks away, a rat dies, a human breaks, a horse splashes."
Terminal Velocity... Here kitty, kitty,...jose_Tex_mex
Oct 16, 2002 12:37 PM
... I do not know the quote you are referencing. However, it might be similar to a study I saw on TV with cats falling out of high rise windows.

Studies have shown that cats falling from just a few stories up were injured more than those that fell from a greater height, to a point. Do not try this at home.

The scientists studied the greater heights and determined that terminal velocity was reached during falls. However, at the lesser heights terminal velocity was not achieved.

The conclusion was that at the greater heights the cats relaxed and were thus able to minimize injury.

Perhaps, the mouse has a lower terminal velocity vs surface area ratio. One thing is for sure - the mouse will spend more time at terminal velocity than anyone else.
Oh and the book has at least oneWayne
Oct 16, 2002 7:19 AM
cool little fact that has stuck with me. The acceleration due to gravity on earth is the equivalent of zero to 60 (mph) in 2 seconds!
Doug. A question.Sintesi
Oct 18, 2002 7:12 AM
Did you have to look this up or is all this info just rattling around in your head? If so that is amazing.

BTW, thanks this is an interesting and fun diversion.

PS: Hope you didn't take my insult in another thread seriously, it was really directed at the czar. My sarcasm is admittedly weird and, as you well know, can be taken the wrong way. Apologies.
African bees or European bees?rwbadley
Oct 16, 2002 7:10 AM
Is this van rated for one ton of bees? It sounds dangerously overloaded to me.

Were the bees weighed in a sitting position? Kneeling? Standing? Flying?

I would say the van will weigh the same with the bees in flight, as the displaced air of the bee flapping will push down on the van floor to counteract the missing bee weight.

RW
A Little Clarificationjose_Tex_mex
Oct 16, 2002 7:44 AM
Suppose the van was at a weigh station. In case A the bees are sitting on the ground. In case B the bees get angry and start buzzing around. In both cases the van is sitting on the scale. If only two readings are made A: when the bees are sitting and B: when the bees are in flight, what will the weight of the van be? Will it increase, decrease, or remain the same?

Answer to follow soon.
A Little ClarificationClydeTri
Oct 16, 2002 7:52 AM
if the engine is running, the weight (mass) is decreasing...not to muddy the waters!
andEager Beagle
Oct 16, 2002 7:54 AM
if the bees are on the ground, are they not in the van?
also...ClydeTri
Oct 16, 2002 7:54 AM
if the sun is out, the van is heating up, causing the air to have less density..thus changing the mass, assuming their is room for the air to expand/dissipate.
dont forget..ClydeTri
Oct 16, 2002 7:55 AM
wind will affect this also...
althoughEager Beagle
Oct 16, 2002 7:58 AM
you will have to offset the decrease in mass as fuel is burned by the running engine against the deposits of burned fuel in the exhaust system.

Does it have a catalytic converter?
is bee faeces,,,,Eager Beagle
Oct 16, 2002 8:01 AM
small enough to be carried away on the breeze?

What size are the vents on the van/are the windows open?

Too many variables here..
One ton of bees could not find room to sit on the floor....rwbadley
Oct 16, 2002 9:06 AM
The floor space taken up by one ton of bees must be substantial. Anyone know the number of bees/pound? This will tell us the amount of floor space needed for one ton of bees to be 'on the floor' as opposed to piled on top of each other.

This will effect the weight of the sitting bees, as when bees are crammed in together they become hot, when they become hot they 'fan' to cool themselves, when they fan to cool themselves they are not in fact sitting or immobile. This must skew the 'sitting' weight of the bees.

:-) So how do you keep morons in suspense? :-)

RW
1 bee = 1 gram nmDougSloan
Oct 16, 2002 9:34 AM
You're killing me...jose_Tex_mex
Oct 16, 2002 12:45 PM
This is exactly the kind of things my students would say :-)I would have to counter by saying:

a) Perhaps, these bees are very dense.
b) I never specified the van, but did say they were all sitting.

This is probably the hardest thing to do in Physics - constraining the problem. The second hardest is getting the students to understand the constraints. No matter how hard one tries, we always attempt to think of things the way we would in reality. No matter what constraints are put on the problem.
MOST IMPORTANTLY!ClydeTri
Oct 16, 2002 9:21 AM
What would Heisenberg say? He would say you could measure their mass, but not guarentee their location, or vice versa...surely cant' do both!
That would only be true for really really really small bees nmPdxMark
Oct 16, 2002 9:47 AM
I agree with ClydeTri on this point...jose_Tex_mex
Oct 16, 2002 1:07 PM
... the bees are made up of small stuff. If you cannot state where the electron is for a particular atom then what can you state [with confidence] about the sum of the atoms?
Answer Time... Newton's Thirdjose_Tex_mex
Oct 16, 2002 9:25 AM
All,

Remember Newton's Third Law?

If you have a van of bees sitting and a van of bees flying around, the van will weigh the same.

The bees are not weightless as is almost the case in outerspace, but they are flying. They fly because they exert a downward force equal to their weight.

Thus, the weight of the van is the same.
But not if...PdxMark
Oct 16, 2002 9:46 AM
the bees all jump up just before the van goes on the scale - rather than flying. OK, let's make it a cow jumping up... or, from the weight survey, about 40 RBR posters...
True...jose_Tex_mex
Oct 16, 2002 11:09 AM
which is exactly why I posed the question when they were in flight :-)

The follow up would involve grasshoppers...
Again you are wrong...ClydeTri
Oct 16, 2002 10:49 AM
The net mass would be the same in the van, but not necessarily the weight. Remember, mass does equal weight. If some of the bees sat in locations in the van higher than others, if they had the same mass they would have different weight since they are further from the center of the mass they are being measured (weighed) against. Remember , the further you are from the center of the earth, the less you weigh, but have the same mass.
correctionClydeTri
Oct 16, 2002 11:04 AM
meant to say, mass does not equal weight
What? You fail today's quiz :-)jose_Tex_mex
Oct 16, 2002 11:07 AM
You said the mass would be the same but not the weight and then go on to say that mass does equal weight? Please explain what you wrote. If [as you claim] mass is equal to weight then why are they different?

The question said the bees sit on the floor. Nothing about different heights.

Finally, you say that the further you are from the center of the earth the less you weigh - NOT TRUE. The further you are from sea level the less you weigh. As you move from the center of the earth towards sea level your weight increases from zero (at the center) to your 100% of your weight (at sea level). Once you move away from sea level towards outer space your weight decreases.
Actually, he's right, in an infinitesimal sort of way,TJeanloz
Oct 16, 2002 11:33 AM
Assuming that the definition of 'flying' is that the bees hover at some point above the floor, the energy required to counteract the accelerating force of gravity will be minutely less (as they are further from the ground than they were). So, in this case, the van will weigh slightly less with the bees in flight than sitting on the floor.

The question I have though, is: are bees capable of 'sitting'? Standing and flying are the only modes I've ever seen. And on the mass/weight issue, Clyde corrected himself to say that mass and weight are not the same.
Or is he!? :-> <---- evil grinjose_Tex_mex
Oct 16, 2002 12:22 PM
I know what you are saying and agree. However, I think I can successfully counter by saying I did not specifically give a height at which the bees are flying. Agree? Also, as you pointed out I did not define flight.

Could I not say the bees are at a "point" above the ground. A point being something that cannot be measured. An infinetesimal "distance." Two points - can measure, one point - nope.

A better "picky" note would have been to say that since the van is covered the mass of the roof attracts (exerts gravity on) the bees and thus helps them to fly - they weigh less.

As for the difference between mass and weight I think our replies met in cyberspace.
Or is he!? :-> <---- evil grinClydeTri
Oct 16, 2002 1:10 PM
For the bees to be flying, they must be above the ground/floor..you can't be flying unless you are somewhat above the ground/floor..now, you are being picky..did you tell us that the van had a roof? it could be a convertible, eh?
Picky...jose_Tex_mex
Oct 16, 2002 1:38 PM
Now come on Clyde... Be reasonable. Would you not say you were being a little bit picky and/or facetious throughout?

Okay then, I decided to counter by saying that the bees are a "point" above the ground. Not a point as in some x, y, z co-ordinate. But literally, one point above the ground. The point cannot be measured as it is infinetesimally small. However, they do not contact the ground. Are they then not in flight?
If they're not contacting the ground, they are in flight...TJeanloz
Oct 17, 2002 5:25 AM
But they are also above the ground, by an infinitely small amount, and weigh a degree infinitely smaller than they had.
If they're not contacting the ground, they are in flight...jose_Tex_mex
Oct 17, 2002 3:23 PM
..but a point does not have a height. As such there is no difference in the gravitational pull which varies with height.
While this is getting ridiculous,TJeanloz
Oct 18, 2002 4:40 AM
So, you're saying that your 'point' does not have height, and thus could be a point that is the same as the point on the floor? I think that the addition of 'flying' indicates that the point does, in fact, exist somewhere above the ground level. If it were not above the ground level, then the bee could not be described as flying at that point.
Of course it's ridiculous...jose_Tex_mex
Oct 18, 2002 5:17 AM
That's precisely why I brought it out. Physics attempts to simplify real life phenomena so we can perform analysis. You can never fully define all constraints which is what some people take advantage of in trying to muddle the discussion. Some concerns are legit and enhance the question. But others are just a waste of time. eg The variance of gravity on the mass of a bee wrt a foot or two? Come on.

With this in mind, I thought of the concept of a point. A point does not have height. Yet I am stating that the bee is flying at that point. My "point" was to factor out gravitational differences by using a meta-physical arguement.

For every action there's an equal and opposite reaction.
Of course it's ridiculous...ClydeTri
Oct 23, 2002 11:15 AM
ah, you made the mistake also, but I suspect you know the difference, as I did..you said "variance of gravity on the mass" Gravity doesn't vary the mass, it varies the weight.
woo! woo! (end zone dance) But if the windows were open...rwbadley
Oct 16, 2002 12:11 PM
and the displaced air from under the wings were allowed easy escape, and the inrush of air from over the wings were easily supplied, the >weight< of the van (+ bees)would decrease, as the path of least resistance is air movement, as oppposed to van floor pressure.

Even tho' it may pencil out in some respects, I am still skeptical of the outcome 'no net change to measured weight of van'
I was wondering about that open window thingDougSloan
Oct 16, 2002 1:28 PM
If the windows are open, the van does not "know" whether the bees are inside or not. There is no pressure on the floor of the van, as the pressure can "escape" through the windows.

Plus, when the bees are flying, isn't the increased pressure under the wings balanced by the decreased pressure over the wings? There is no net force, right?
Now, can we work Einstein into this discussion...ClydeTri
Oct 16, 2002 1:16 PM
So, according to Einstein, as the bees velocity increases, their mass increases, so, a van full of flying bees has more mass than a van full of "static" bees. YOu didnt say how fast they can fly, and how big this van is. The van could be quite long and they could be flying from end to end at high velocity.
One thing you are forgetting,...jose_Tex_mex
Oct 16, 2002 1:30 PM
Okay Mr. Clyde, that's it. I am calling your parents :-)

Actually, I have a kid in one of my classes who enjoys cranking me up in much the same fashion as yourself - I enjoy it as it keeps me thinking and shows they are putting in the extra effort.

HOWEVER, :-) one thing you are forgetting is that this is a physics question. You are blending Physics with MetaPhysics. Much as Einstein would not say where one stopped and the other started, neither will I :-) Nice cop out, huh!!!

I will say that the question posed went to measurements. I think I agreed with most of what you have to say (in a metaphysical point of view). However, the original question did ask for a measurement - weight. You cannot measure the differnces you stated and as such the weights in the two cases would be equal.

What you said about Einstein is true, as the bees move they are not only changing their mass but time traveling as well. As a matter of fact their wings are time traveling with respect to each other, on a very, very minute scale.

All you have to do to prove me wrong is find a way to measure the differences you mentioned.
Trick questionKristin
Oct 16, 2002 2:23 PM
Yes, the VAN when the bees are sitting on the floor is heavier than just the bees. Did you mean to word it that way?
Not supposed to be...jose_Tex_mex
Oct 16, 2002 2:45 PM
...it was just supposed to illustrate Newton's Third Law. Most people would say that the van weighed less when the bees were flying. However, since the bees are exerting a force downward in order to fly, the weight of the van remains the same.

Then we went off on tangents :-)
I think you are misapplying Newton's Third LawDougSloan
Oct 17, 2002 6:22 AM
>For every action there is an equal and opposite reaction.

Doesn't this mean that the downward pressure of the wings are balanced by the upward negative pressure above the wings? Hovering or flying level, the forces are balanced, exerting no net force on the van.

Plus, what is the mechanism for applying any force to the floor of the van while the bees are in flight? Air pressure? Air is a fluid (for these purposes), and fluids exert pressure equally in all directions, don't they? The pressure on the floor of the van is (to any significant degree) the same as the pressure on the ceiling, balancing each other out.

Doug
let's look at one bee...jose_Tex_mex
Oct 17, 2002 9:31 AM
The air is supporting the weight of the bee. Thus, the weight needs to be counteracted with an upward force of equal magnitude and opposite direction - Newton's Third.

If the air provides an upward force on the bees, it feels an opposite force of the same magnitude pushing down on it. The downward force on the air results in a downward force on the floor of the van equal to the weight of the bees.

Thus, a person outside of a closed van cannot tell whether the bees inside are in flight or sitting just be the scale.
no, still not convincedDougSloan
Oct 17, 2002 9:44 AM
I think you are neglecting to account for the physical properties of air. Assuming the bee exerts additional pressue on the air below it when flying, what happens to that pressure?

The pressure is NOT distributed solely downward in a vertical plane against the van floor. The air is not a solid column supporting the bee. Rather, if there is increased air pressure supporting the bee, that pressure is evenly distributed against every interior surface of the van, if the van if air tight. That is how fluid pressure acts. Therefore, there is as much pressure on the ceiling of the van as there is the floor (or sides), meaning there is no net force on the floor of the van.

Open the windows, and any increased pressure is released, and the van floor or any interior surface of the van has no increased pressure upon it.

If I'm missing something, I'd sure like to know it. Thanks.

Doug
One simple question...jose_Tex_mex
Oct 17, 2002 3:00 PM
..how can the bee push up if there is not an equal and opposite force?
Do standing kangaroos weigh more than leaping kangaroos?nn23
Oct 21, 2002 12:57 PM
Does a truck with a kangaroo standing in the back weigh more than when the kangaroo is in flight ? Think of a small kangaroo in a HUGE truck at a weigh station.

Don't bother answering ;)
So I'm crushed when a 747 flies over me?DougSloan
Oct 17, 2002 6:13 AM
I think the conclusion the van does not weigh less when the bees are in flight is wrong. Change the scenario to this: The bees are sitting on the roof of the van instead of the floor. They then start flying, but hovering above the van. Does the van weigh less? Of course it does!

If you don't believe that, imagine this. You are standing outside. A 747 flies directly over you. Do you get crushed from the plane? Of course not. The plane flying adds no weight to things below it. The plane, and all things flying, balance increased pressure below the wings with decreased pressure above. That's why things fly.

Doug
I think Doug is right....Wayne
Oct 17, 2002 6:44 AM
the van weighs less when the bees are in flight. I kept thinking of the same type of analogies that you propose. I don't think most animals fly by exerting a pressure equal to their weight against the underlying solid surface.
here's another wayDougSloan
Oct 17, 2002 7:02 AM
Think of this. The bees are sitting on a scale on the floor of the fan, which is recording, let's say, 1 pound. The scale, then, is applying a 1 pound (plus the weight of the scale) force to the floor of the van. The bees then start flying. Does the scale continue to show 1 pound? (let's say the scale covers the entire floor of the van) We know darn well the scale will then read zero. If the scale reads zero, it then is applying less force to the floor of the van.

I think what's being ignored here is that a net force is being applied by the bees -- energy is being used to lift the bees. This energy is opposing the force of gravity, making the bees "lighter".

Doug
I bet you exert more pressure standing on the ground...jose_Tex_mex
Oct 17, 2002 10:19 AM
...than the plane experiences in flight. I will search the internet for the surface area of the plane and get back to you on this. What you are forgetting is that the plane's surface area is enormous. Do you think the mass of the plane is greater than atmospheric pressure? If the plane's pressure is much greater than atmos than one would expect to feel it. Otherwise, it might not add much.

Also, I will need to get standard atm pressure in order to determine how much a column of air weighs. Once, I know the weight of a column of air say 1in^2 in surface area I will be able to compare it to the weight of the plane. I think a one inch square on the surface of the earth has a column of air above it of weight 15lbs. I'll have to double check.

Also, I don't think we are comparing apples to oranges any more. We have gone from the case of bees inside a van and are now attempting to apply that logic to a different form of flight under different conditions.

I don't have time to work it all out, but I will get back to this ASAP.
Part One - How much does our atmosphere weigh...jose_Tex_mex
Oct 17, 2002 3:52 PM
Actually, I guess we really do not need that calculation since I found a value for our atmospheric pressure - 14.7lbs/in^2

In case you are wondering the weight of the atmosphere, is 6x10^15 tons. If anyone is interested in the calculation let me know and I'll write it out.

So atmos pres is 14.7lbs/in^2 this means that at the surface of the earth if you had a one inch square area and projected this area upward, you would have a column of air. The weight of this column of air is 14.7lbs. Thus, the atmosphere acts on a square inch of your head with a force of about 15lbs. Doesn't feel like you have a 15lb weight on your head does it?

I am going to check about the plane now. Although, again it's comparing apples to oranges.
The Simple Science of Flight : From Insects to Jumbo Jetsjose_Tex_mex
Oct 17, 2002 4:35 PM
The Simple Science of Flight : From Insects to Jumbo Jets
by Henk Tennekes

Please credit the above author with the following two paragraphs of data which I knew I read once upon a time. Great book BTW.

For the Boeing 747 the surface area of the wings is 5500ft^2 and has a lifting capability of up to 800,000lbs. Simply dividing gives us a carrying capacity of 145 lbs/square foot. Sounds like a lot huh?

Okay, how about your 150lbs acting upon the 30 square inches of your shoes? Whoa! 750lbs/ft^2. That's 5 times than that off a jet's wings. Take your sweetie and measure the surface area of her high heels and watch the pressure jump to 20,000lbs/in^2.

Let's try comparing the max rating for the wings to that of atmospheric pressure.

Atm Pres - 15 lbs/in^2
Wing Pres - 145 lbs/ft^2

Changing the Wing Pressure from ft^2 to in^2 requires us to multiply by (1ft^2/(12)^2 in^2) - that is, there are 144 square inches in a square foot. This makes the force on the wings 1lb/in^2.

Thus, if the plane were static and hovering somehow like the bee I would argue that every square inch of the wing would push down on a column of air (of cross section 1in^2)with a pound force.

Since the atmosphere pushes down with 15 pound forces I doubt you would notice the extra one. Although, who knows. Keep in mind this is the most force the 747 can push down with.
how about this?DougSloan
Oct 17, 2002 7:14 PM
A Saturn V rocket takes off, then turns and goes vertical again right over your head (but high enough not to incinerate you). With a diameter of 30 feet, 7.5 million pounds of thrust and a take off weight of over 6 million pounds, it would surely crush you, right?

Doug
Watch that "surely" stuff...jose_Tex_mex
Oct 18, 2002 5:10 AM
It's a lot like when people say something is obvious.

Again we're changing constraints, however, I'll play along. The big change here is that in the case of the Saturn V, the rocket needs to carry its own supply of oxygen. Now we are not looking at just the craft but that of the craft + oxygen. That's a little unfair since neither the bee nor the plane have this constraint.

Both planes and rockets function because of Newton's Third Law - especially the rocket when in outer space. Both eject combusted gasses in opposition to the desired direction of acceleration.

Now let's suppose the Saturn V had a weight of 5x10^6lbs and a diameter of 33ft - as per Boeing - close to what you said. Also, let's say that the rocket was capable of hovering similar to the bee while maintaining its weight as a constant.

The radius is 16.5 ft and the area of this cross section would be Pi*r^2 about 855ft^2.

The force the rocket exerts on this area is 5E6lbs/855ft^2 which is about 5848lbs/ft^2 or 40.61lbs/in^2 that's about 2.7 times atmospheric pressure.

The atmospheric pressure of 14.7lbs/in^2 can also be described as one atmosphere (1atm). Thus, if we were under the rocket hovering close to the ground we would feel 2.7atm.

Question is: would this crush us? Well if we were to dive to 66ft the absolute pressure would be 3atm. I would answer no.

Since water is pretty much incompressible the density of water remains constant unlike that of air. Thus, we can continue the pressure trend to predict the pressures free divers can withstand. A quick google search noted a diver reached a depth of 593ft which would have put about 20atm on the person.

I think you are highlighting the contoversy wrt what makes things fly - Newton or Bernoulli?
So I'm crushed when a 747 flies over me? - Yes.nn23
Oct 21, 2002 1:26 PM
If the 747 were to fly only a few feet above you, you will feel the downward thrust as a strong wind. The higher the 747 is from the earth, greater the surface area over which this downward thrust would be averaged out. This would be small enough in comparison with atmospheric pressure (14.7 lbs per sq inch) and its fluctuations so as to be virtually immeasurable.

"The plane, and all things flying, balance increased pressure below the wings with decreased pressure above."

Yes ... and the increase in pressure below will show up as weight if the machine can account pressure difference also. (Most cannot)

When water from the oceans evaporates and becomes clouds/vapor, does the weight on the surface of the earth reduce, or does it just transtate to higher atmospheric pressure on the surface?
So then, which is heavier:Kristin
Oct 17, 2002 8:49 AM
A van contains one Harrier Jet sitting on the floor. Which weighs more - the van when the jet is resting on the floor or when it is flying inside the van?

Bonus question: How long before the van disintegrates?
Oops, forgot the visual aidKristin
Oct 17, 2002 8:50 AM
This would be fun to ride through rush hour.
Kristin's Example is PERFECT!jose_Tex_mex
Oct 17, 2002 10:22 AM
Great analogy Kristin. Does anyone not see that the weight of the van with the harrier at rest is equal to the same weight when the harrier is hovering?

The Harrier hovers because it is supported by the air. There's an equal and opposite force on the floor of the van equal to its weight.

Would anyone like to be under the Harrier?
You're assuming that bees and Harrier Jets...Wayne
Oct 17, 2002 11:35 AM
hover (fly) by the same mechanism, and I don't think that is true.
Thank youKristin
Oct 17, 2002 11:52 AM
I posted this as a joke of sorts. Hovers achieve lift by creating a downward air mass thru rotational jet engines. (So, I believe, the weight of the van with the hover hovering would be even heavier than the weight of van with the hover resting on the floor. Because the amount of downward force needs to be greater than the weight of the aircraft in order to lift it off the ground, no?)

Bees don't use downward force. Neither do airplanes. They use lift.
right; Harriers don't fly when hoveringDougSloan
Oct 17, 2002 1:54 PM
Harriers use pure thrust when hovering; they are not flying, as there is no aerodynamic lift; they might as well be considered rockets; if there were no air at all around them, except to feed the engines, they would still hover.

Didn't anyone ready the post above I did that contains a passage about the "bees not being able to fly" article? One method of lift bees use is to flap their wings together above their bodies, so that when the wings separate, it creates a vacuum, helping to lift the bee. That would tend to lighten the bee, wouldn't it (and anything it's supported by)?

Doug
can a bee fly in a vacuum?jose_Tex_mex
Oct 17, 2002 3:42 PM
Nope, they need the air to provide the upward force.

Also, doesn't a Harrier mostly use air to thrust and rockets ignited gasses? How useful would a Harrier be in outer space? Not very, if at all. If you put a Harrier in to a vacuum chamber here on Earth it wouldn't get off the ground.

Thus, a Harrier is not much of a rocket as it relies on its ability to vector thrust - force air in a given direction for movement. It uses Newton's Third law to get to a point whereby it can gain forward momentum and eventually lift.

Finally, I doubt if the study you mentioned really meant to say there was a vacuum over the bees wings. It's more likely they meant a partial vacuum much the same as when cyclists receive benefits from the partial vacuum the leader leaves behind.
we are passing in the night, hereDougSloan
Oct 17, 2002 4:30 PM
Of course the bee needs air, as does the harrier. The point about the harrier is that the air is used essentially as propellant, not as a fluid through which to fly. If it carried big compressed air tanks, it could do the same thing. That's what I mean when I say it's like a rocket. It's thrust exceeds its weight, unlike most airplanes.

What you have not addressed is what happens to the air (fluid) the is so slightely compressed by the bee when flying? Again, I say that the air pressure is equally distributed on all surfaces of the enclosed van. What about that?

Doug
sounds like a lot of vectors will cancel...jose_Tex_mex
Oct 17, 2002 4:39 PM
... and that I will be left with the weight of the bee acting downward. Knowing that the bee is hovering I will say that the air is supporting the bee by exerting an equal and opposite force upward.
But...jose_Tex_mex
Oct 17, 2002 3:12 PM
...the harrier is not flying and thus not creating lift. It is hovering. Notice, the question only looked at the state of the bees/harrier at rest and then when they were in the air. I purposely avoided the case where they were getting to their hover position.

So the harrier wants to fly. How much thrust does it need to hover. If it thrusts greater than its weight does it not rise? If it thrusts less than its wieght does it not fall? If it stays in the middle, its thrust is equal to its weight.
no such assumption by their mechanism...jose_Tex_mex
Oct 17, 2002 3:07 PM
..just that both hover. Again, both objects are being supported by air and are definitely not weightless. Since they are hovering their respective weights needs to be counteracted with an equal and opposite upward force.

Newton's third law of motion also says that if the air provides an upward force, it feels an opposite force of the same magnitude pushing down on it.

The downward force on the air results in a downward force on the floor of the van equal to the weight of the bees or the harrier.

That's my story and I am sticking to it :-)
you are right but....Bruno
Oct 18, 2002 2:40 PM
there is no "column" of air. The air that is being push down by a rocket, helicopter, or the wings of an airplane disipates in a large volume. This is the reason a change in pressure wouldn't be felt on the ground. (except if you are just a few feet from the aircraft). Instead of a column think about the air affected by the change in pressure as a cone.