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> bird flying in a plane, does the plane weigh more?
rastetter
Posted: Jun 26 2006, 03:45 AM


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lets say you have a 747 with an eagle in it

the eagle weighs lets say 100 pounds... If the eagle starts to fly while inside the plane, will the plane weigh less?

This is confusing me, technically the wings of the birds down force would push on the plane making it weigh more

but then lets say you have a little remote control plane flying in a 747, does that add weight?
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NickFun
Posted: Jun 26 2006, 04:07 AM


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If the plane is pressurized I would think it would weigh more. Unpressurized, no. But I don't know for sure.
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rastetter
Posted: Jun 26 2006, 05:41 PM


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I dont think pressure in the cabin makes a difference
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calnpals
Posted: Jun 26 2006, 05:52 PM


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Yes, it would weigh more, since the plane is enclosed (I'm assuming it is), the mass of the plane will not change, thus the weight will not change. If the plane WASN'T enclosed, then some of the wind blown from the wings might just go out the openings, thus making is lighter than if the bird was sitting.

In an enclosed area, the bird will keep itself up by pushing against the air, which will in turn push against the plane, no weight change.
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krreagan
Posted: Jun 26 2006, 06:15 PM


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The bird causes a downward force equal to its weight in order to stay alloft. This force is transmitted by the air to the enclosed fuselage of the plane and on to it's wings... so the plane needs an extra lift for the extra weight of the bird (a very minor extra lift in the case of a 747 smile.gif.

In the case where the plane is not enclosed some (but not all) of this force can be transmitted to the ground through the normal means (atmosphere), as all birds exert an external force to say alloft. All forces have to be accounted for. How much force is felt by the plane is dependent on the physical configuration of "open to the atmosphere". If a bird is fling inside an open bomb bay of a b52 say, then most of the force will be transmitted to the ground. Now if the doors are closed most (if not all) of the force will be transmitted to he plane.

A pressurized plane weighs a little more then an unpressurized plane due to the increased amount of atmosphere in the plane. (there's more air in the plane when it's pressurized)

Krreagan


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"It is impossible to defeat an ignorant man in argument." [William G. McAdoo]
"Against logic there is no armor like ignorance." [Laurence J. Peter]
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rastetter
Posted: Jun 27 2006, 04:29 AM


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what about a small remote control airplane inside a 747?
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calnpals
Posted: Jun 27 2006, 05:11 PM


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Same thing. Plane pushes on air(small plane), air pushes on plane(big plane). No weight change.

Now the question is, if you threw a bowling ball in the air, would the weight change when the ball is in the air compared to when it's in your hands?(again aboard the enclosed 747)
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Nick
Posted: Jun 27 2006, 08:24 PM


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This is similar to light in a box. Since light falls it would be weightless and could only add to the weight of the box after it has been absorbed by the walls.

Light is weightless. It is always in freefall.
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rastetter
Posted: Jun 27 2006, 09:18 PM


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well if you through a bowling ball while in a plane, as you accelerate the ball to get it in the air, the plane would weigh more. When you release the ball it would weigh less, and when it hit the ground, the plane would weigh more... but over that whole time span the planes weight on average is the same

it would be like jumping up and down in the plane
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mr_homm
Posted: Jun 27 2006, 09:53 PM


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Well, technically, the weight of the plane is defined as the force of gravity acting on the plane. The bird, whether flying or not, is irrelevant. You could fill the plane with 1000 tons of lead, and the PLANE wouldn't weigh any more.

Now the system consisting of the plane and bird is probably what is meant. Again, the weight of the system is defined as the force of gravity acting on the system. Whether the bird is flying or not, gravity continues to pull on it, just as it does on the plane. Therefore, the SYSTEM wouldn't weigh any more.

Sometimes people confuse weight with what a scale would read, which is actually the normal force exerted downward on the scale. By Newton's third law, this is equal in size to the force the scale exerts upward on the object. By Newton's second law, this is equal in size to the force of weight on the object, PROVIDED that there are no other vertical forces on the object and also provided that the object has no vertical acceleration.

If we imagine the plane standing on a scale on the ground, then whether the bird is flying or not, the bird has no vertical acceleration, hence nothing in the system has vertical acceleration, since the plane certainly doesn't. If the sole force supporting the system is coming from the scale, then the scale reading will be the same whether the bird flies or not. However, at the moment the bird jumps up and starts to fly, there is temporarily a small upward acceleration of the bird, followed by a small downward acceleration as the bird reaches its height of level flight and stops rising. Therefore, briefly, the scale will show very slightly more weight and then very slightly less weight as the bird begins to fly. The same will happen in reverse when the bird lands.

If we imaging that the plane is flying (level, steady flight), then the upward lift on the plane's wings is doing the job of holding up the plane, so you can treat this as if it were the scale's force. Therefore the same conclusion applies: just as the bird takes off, the lift will first slightly increase, then slightly decrease, but afterwards, when the bird is airborne within the plane, it will return EXACTLY to its original value. However, lift is not weight, so this has nothing to do with the weight of the plane changing.

If the plane is open to the outside air, instead of enclosed, then it is possible that some of the air forced down by the bird's wings will escape from the plane. In that case, the lift of the plane will decrease slightly while the bird is flying. This is no surprise, since the bird is partly supporting its own weight now. If all the air forced down by the bird's wings escaped from the plane (while still moving downward), then the bird is fully supporting its own weight, which is the situation you have when the bird is flying along outside the plane, beside it. But again, this has nothing to do with the plane's weight, only with its lift.

--Stuart Anderson


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rastetter
Posted: Jun 28 2006, 05:51 AM


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what about with a remote controlled airplane, something that doesnt use downward force to generate lift?
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nautilus
Posted: Jun 28 2006, 01:40 PM


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I cringe every time someone mentions "plane" on here. tongue.gif

Wouldn't the remote controlled airplane do the same thing? Just push air around as it flies, so the weight wouldn't change overall.


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rastetter
Posted: Jun 28 2006, 02:09 PM


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but i planes propellor doesnt need to generate as much thrust as a helicopter of the same weight
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calnpals
Posted: Jun 28 2006, 02:20 PM


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QUOTE (rastetter @ Jun 28 2006, 02:09 PM)
but i planes propellor doesnt need to generate as much thrust as a helicopter of the same weight

Good point, but the plane also does something else that the helicopter doesn't do, it moves. The whole point of this question is to decide if the displacement of air that any object uses to keep itself afloat will change the weight in an enclosed environment. The answer is no, the object pushes along the air displacing it, which in turn pushes on the enclosed space.

When a plane moves, the added air that it displaces while moving equals that of the displaced air of any other object, be it helicopter or bird.
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rastetter
Posted: Jun 30 2006, 04:04 PM


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well then as the remote control plane goes faster and faster the 747 would actually get heavier then if it was not moving?
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