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Author Topic: If a box of pigeons flap their wings and take off, does the box lose weight?  (Read 8965 times)

Offline Karsten

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This is a problem my father faced me with when I was a kid. We never came to an agreement, and while I have reached an opinion about this in the meantime, I would like to run the scenario by you and get some feed-back:

A box truck loaded with 2000 pigeons arrives at a bridge. The bridge is limited to slightly less than the truck weighs at the moment. The driver gets out with a stick, pounds on the outside of the truck to scare the pigeons and get them flying. He then drives the truck quickly across the bridge while the pigeons are still airborne. The truck got lighter because the pigeons are flying inside the truck. Does that work?

I think it does, but how about you? Why? Why not?

Curious,
Karsten
« Last Edit: 06/01/2009 18:13:46 by chris »


 

Offline BenV

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I don't think it would work, as the pigeons must be pushing down on something in order to gain lift.  However, if they were in freefall inside the lorry it may be different - any physicists care to help?
 

Offline DoctorBeaver

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I don't think it would work, as the pigeons must be pushing down on something in order to gain lift.  However, if they were in freefall inside the lorry it may be different - any physicists care to help?

I agree. Equal and opposite reaction, and all that. Then again, I'm just a dumb beaver so don't listen to anything I say.
 

Offline graham.d

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Quite right if you think of box truck as a sealed box; all that can happen is that its weight will fluctuate but the average will stay the same. However, if instead of a sealed box we have a cage then the issue is rather different. An extreme example to illustrate this would be to consider the truck/cage to be moving at an ideal flying speed for the pigeons and then using highly trained pigeons to simply fly forward at that speed. The effect on the truck would only be the same as if pigeons were flying above an empty truck. I am assuming that the chassis of the truck could be made insignificant in absorbing too much effect from any resulting turbulance as this would be a matter of design rather than a fundamental issue.
 

Offline LeeE

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I don't think it would work, as the pigeons must be pushing down on something in order to gain lift.  However, if they were in freefall inside the lorry it may be different - any physicists care to help?

Most of the lift from a wing doesn't come from pushing down on something but by creating low pressure regions above them, so it's more a case of being sucked upwards.  Instead of the birds creating a thrust downwards, which would act upon the lorry, they would be creating a lots of low-pressure regions, which would tend to suck the roof down.  In practice though, with so many birds in such a small closed space, the turbulence created by each bird would upset the conditions for the birds next to them and make flying more difficult.
 

lyner

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He should tie them all with long ribbons and let them fly free. A bit of training could get them all flying in the same direction.
A bit like the guy with all those helium balloons!
 

Offline Madidus_Scientia

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I think it would work, because as LeeE says, the birds are essentially sucking themselves upwards, not pushing downwards on the truck. The birds are relying on their own force generated by their muscles to counteract gravity, rather than just resting on the truck.
 

lyner

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the birds are essentially sucking themselves upwards
A dodgy statement. There has to be a pressure difference for a resultant force. Gas can't suck because the molecules aren't attached to each other to produce tension but a higher pressure below can produce a net upward force.

The problem with this scenario is that birds are designed to work in isolation (or when slipstreaming). If they are too close together, the airflow will be all wrong - turbulent and not laminar- and produce incredible power losses.
 

Offline Madidus_Scientia

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the birds are essentially sucking themselves upwards
A dodgy statement.

You should have said that it sucks :P I see where my thinking went off track now, thank you.

All of the force couldn't be directed exactly downwards though could it? What if you had a set of scales the same radius as that of a helicopters blades, and a helicopter hovered or flew directly over them high above, would the scales register the helicopters weight?

The force would disperse out just like a sound wave would it?
 

Offline Karsten

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Reading the comments above has been great fun so far. Much has been mentioned that my father and I have discussed. I completely forgot about the argument of the birds flying over the truck in contrast to them flying inside of the truck.

If, like some seem to say here, the truck experiences being pushed down as hard as the force that is required to lift the bird, would one not conclude that the same phenomenon would occur at the ceiling of the truck. Higher pressure below the bird pushes the truck down, AND lower pressure above the bird sucks the truck down as well. How can you create higher pressure in a container only on one surface(= the floor)? If an airplane flies under a high bridge, does the bridge get sucked down and the ground pushed down as well?

Here is the hypothesis (if you can even call it that) I have cooked up over the years:

The birds will be flying. Since they are not hovering close to the ground but are engaged in forward flight at a reasonable height of the floor, the lift necessary to lift them of the ground (= truck) is generated by moving their wings in a fashion that creates a low pressure area above the wings, and high pressure area below the wings. This pressure difference lifts the bird but is equalized immediately after the wings have flown through the air. Basically at the trailing edge of each wing the pressure is the same again. The energy to lift comes from the pigeon itself. If the birds spend energy to lift themselves by creating a pressure inequality, and the air does not get pushed down equally hard (I am assuming no ground effect), the truck should get lighter. Am I wrong? Maybe I should not compare flapping bird wings with fixed-wing aircraft.

If I stand in a truck on a bathroom scale with a hawk on my arm and the hawk flies off, what happens? Does the scale show that I am lighter? I think so. And the truck would not show that it is lighter now?

What do you think?

Karsten
 

Offline DoctorBeaver

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If I stand in a truck on a bathroom scale with a hawk on my arm and the hawk flies off, what happens? Does the scale show that I am lighter? I think so. And the truck would not show that it is lighter now?

Your weight would momentarily increase as the bird jumped into the air.
 

Offline BenV

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The birds will be flying. Since they are not hovering close to the ground but are engaged in forward flight at a reasonable height of the floor...
Wouldn't that assume that the air inside the truck isn't being being carried along with the truck?  Otherwise, they couldn't fly more than the length of the truck, would mainly be hovering and this would be a very different situation to a bird flying above the ceiling.  It's just practicalities, but I've always pictured this as being a (relatively) sealed truck (i.e. cloth sides at least), rather than a cage.
 

Offline graham.d

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Karsten, I refer to my previous answer. If the truck is a reasonably sealed container (it can have leaks and airflow to allow the birds to breathe) the truck's weight will fluctuate but the average will stay the same. If you can devise any scheme which involves a sealed container but somehow can change its weight (for a lengthy period) by some event inside the box, you would make a huge amount of money!
 

Offline Karsten

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Graham, this is EXACTLY the answer! You are right. If the box is sealed and the pigeons make the box lighter, I (and most likely someone a long time ago) would have made a boat load of money. An event inside made the box lighter. Anti-gravity! Who would have thought it is so simple.

Oh well, my thinking was wrong assuming it is a sealed container. But how about a huge wire cage instead of a sealed container. Room enough for 1000 finches to fly in circles. Does it get lighter?

And, who changed the subject line of the thread? Just curious. Why?

Thanks,
Karsten
 

Offline Chemistry4me

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And, who changed the subject line of the thread? Just curious. Why?
The moderators want the forum questions to be set out like questions (with a question mark).
 

Offline Chemistry4me

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Same with your "Human electromagnetic wave modulator" topic, they'll probably rearrange it to form a question, unless you want to do it yourself now.
 

Offline Karsten

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Ah, thank you. I will do that in the future.
Karsten
 

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