[Joe L. Ogan]

Momentum is a conserved property.
If you blow across the room you can make the curtains move because they are big and intercept a  lot of the air you moved by blowing.

It's really very simple- the fly is held up by the air. The air is held up by the plane and if the plane were held up by a set of scales then they would show the fly's weight as well as the plane's.
What else could be holding

The airpressure on the oustside of the plane nullifies the weight of the air inside the airplane unless the inside is pressurized!  Thanks for comments.  Joe L. Ogan

[CZARCAR]

i can stand on a truckbed. when i jump up, the bed goes down. when i land the bed goes down.= dead fly aint gonna make a diff, live fly will burn fat to fly& unless its pushing on the ceiling of the plane?

[peppercorn]

Ok, say I have a really large (sealed) room, and I repeat the experiment in it, what would happen?

Well, as a closed system (assuming thermally closed as well) I guess the only place for any of the chemical energy stored in the fly (as it flies, hits floors, etc) is into heat - warming the air temp infinitesimally.

[CZARCAR]

The fly in order to fly has to burn its flyfat. The fly flies up into the ceiling of the plane with as much force as it delivers to the floor of the plane as it drops after hitting the ceiling. The fly gets lighter as it converts flyfat to energy...the fly is flying against the force of gravity?

[elfabyanos]

The fly does add to the weight of the system. Plane + air + fly + anything else that is inside the plane = weight of plane system.

The only way the fly can change the weight of the plane is by either altering the mass of itself, the air or the plane, or by altering the strength of gravity. The fly cannot do this, therefore it cannot affect the weight.

All the situations proposed do not affect the weight. They affect forces.

If the fly flies up and hits the ceiling the plane will experience a momentary upwards force as the fly transfers the energy of its momentum to the plane. The fly generated that momentum by turning chemical energy into kinetic energy. An equal amount of energy was expended pushing air downwards as was expended pushing the fly upwards. The kinetic energy of the air going downwards is dissipated to a certain extent over time. It will take longer, but a much larger mass of air will hit the bottom of the plane at a much lower speed than the fly which hit the ceiling fast. Nonetheless the result of a small fly going fast and a large amount of air going slowly but in the opposite direction will eventually cancel. That the air and the fly do not transfer the forces at the same time is irrelevent, except that it would cause the temporary wobbles up and down.

The plane will bob up and down slightly over this period, but this will not affect the result. Same as if you stand on a boat and rock it side to side.

Ultimately you or the fly can temporarily upset the system in a number of ways, as can any other element within that system. But that is not a question of weight, which exerts a constant force due to gravity at all times, without variation. It is all the other forces which are invoked by the thought experiment of the fly flying around the plane.

[Joe L. Ogan]

I believe that the fly is still exerting the force of gravity by flying to support his weight.  Thanks for comments.  Joe L. Ogan

[SteveFish]

Rami:

The answer to your plane above a balance question is yes it will record the weight of the plane. For visualization purposes you can exaggerate the situation by making it a helicopter hovering just above the bottom of a large box on a balance. If you were standing just below the helicopter you would experience the high wind striking the floor. When the helicopter flies up very high the strong but small downdraft just below it is distributed, ultimately, to a very broad and deep mass of air moving very slowly that at ground level might be so small as to not be measurable. An airplane and a fly have to thrust a lot of air downward in order to resist gravity. The downward air movement is transferred to the ground, and the downward thrust of air from a fly in an airplane is transferred to the plane.

Steve

[Joe L. Ogan]

If the fly had no weight, there would be no effect.  Since the fly  does have weight there is effect on the overall weight.  Thanks for comments.  Joe L. Ogan

[elfabyanos]

No, you are talking about the force due to the movement of air by the flying object, and I do agree with you that that force will affect the experiment, but my question was will the balance record anything due the object's weight only, not the air molecules.
 

Well ask yourself this, how does a balance work? By direct contact and direct contact only. The answer is no, the balance will not record any of the plane's weight if the plane is not in contact with it. Or the fly for that matter, if the balance is inside the plane.

What it will record is the forces created by air movement hitting it.

However, I am unsure what it is you are trying to say, a fly and/or plane does not change weight. If a fly is sealed in a plane, the plane's take-off weight is fly + air + plane. This is because the mass of fly + air + plane is constant, and so is gravity.

The reason why the balance or scales will not record the weight of an object above it but separated by a gas (or liquid) is due to the way gases and liquids distribute pressure. The entire weight of the air (or liquid) and all the objects within it is distributed equally on all horizontal surfaces that support the air / liquid. This is a fundemental property of the states of liqid, gas, and to a certain extent plasma.

I think where you are confusing yourself is by expecting gas and liquid to behave like a solid, and transfer forces directly downwards in an obvious way.

[SteveFish]

Elfabyanos:

You are confusing pressure with movement of the air. Pressure is distributed equally via random molecular movement, but when you move a mass of air there is a molecular movement, and pressure bias in the direction of the movement. Otherwise you couldn't enjoy the breeze from a fan because it would be distributed in all directions about the fan and decrease by the inverse square law. It doesn't. The very large movement of air directed downward from an airplane in flight is distributed, ultimately, to the ground, and a fly, in flight, in an airplane, distributes its weight to the floor of the plane cabin.

Steve

[CZARCAR]

Elfabyanos:

You are confusing pressure with movement of the air. Pressure is distributed equally via random molecular movement, but when you move a mass of air there is a molecular movement, and pressure bias in the direction of the movement. Otherwise you couldn't enjoy the breeze from a fan because it would be distributed in all directions about the fan and decrease by the inverse square law. It doesn't. The very large movement of air directed downward from an airplane in flight is distributed, ultimately, to the ground, and a fly, in flight, in an airplane, distributes its weight to the floor of the plane cabin.

Steve

what if the fly is persistent on flying & applying that force to the ceiling of the airplane?

[tbarron]

Another way of thinking about it would be to imagine a big tub of water on a scale. Suppose the tub is full to the brim. If we drop in a frog that starts swimming around in the tub, does the weight registered on the scale increase?

If the tub is full to the brim, the water displaced by the frog will slosh over the side of the tub. This will exactly match the weight of the frog, so the weight registered by the scale will not change at all. (weight of full tub of water + weight of frog - weight of water displaced by frog = weight of full tub of water).

On the other hand, if the tub is not full, so the water displaced by the frog stays in the tub, the weight registered by the scale will increase by the weight of the frog.

But this implies that the plane-and-bird system above will weigh P (weight of plane) + B (weight of bird, or fly), even if the bird is in flight inside the plane. Thinking about the frog in the tub, suppose the tub full of water weighs T. We add the frog, which weighs F. The tub with the frog swimming it it weighs T + F. If the frog swims over to the side of the tub and climbs up on the edge, the system still weighs T + F. The weight of the system doesn't change depending on whether the frog is swimming or not, so the weight of the plane and bird system shouldn't change just because the bird is flying.

[tbarron]

Elfabyanos:

You are confusing pressure with movement of the air. Pressure is distributed equally via random molecular movement, but when you move a mass of air there is a molecular movement, and pressure bias in the direction of the movement. Otherwise you couldn't enjoy the breeze from a fan because it would be distributed in all directions about the fan and decrease by the inverse square law. It doesn't. The very large movement of air directed downward from an airplane in flight is distributed, ultimately, to the ground, and a fly, in flight, in an airplane, distributes its weight to the floor of the plane cabin.

Steve

what if the fly is persistent on flying & applying that force to the ceiling of the airplane?

Someone suggested that this is like rocking a boat from side to side. We might also say it's like bouncing up and down in the boat to cause up and down oscillations. To get momentum to fly into the ceiling, the fly has to push air downward. That pressure is transferred to the plane and exactly cancels the effect of the fly's momentum hitting the ceiling.

[elfabyanos]

Elfabyanos:

You are confusing pressure with movement of the air. Pressure is distributed equally via random molecular movement, but when you move a mass of air there is a molecular movement, and pressure bias in the direction of the movement. Otherwise you couldn't enjoy the breeze from a fan because it would be distributed in all directions about the fan and decrease by the inverse square law. It doesn't. The very large movement of air directed downward from an airplane in flight is distributed, ultimately, to the ground, and a fly, in flight, in an airplane, distributes its weight to the floor of the plane cabin.

Steve

Yes exactly thats why I said its to do with the way in which the horizontal surfaces that support the weight, but I think I phrased it badly. I meant the pressure felt by the receiving surface ie the floor - from the floors point of view there is little difference between air pressure and air movement, it has the same effect although air movement can cause an angled vector of the force whares pressure is always perpendicular.

The thing I was trying to get at is that small movements of air over a distance will dissipate to an extent where there seems to be no actual movement, but there is a tiny amount but over a much bigger area so the total force is the same.

[SteveFish]

Elfabyanos, I understand. I think I even said something similar way up there somewhere. Steve