A 500 pound fly (this is not a HW question)

[Xelb]

So this is a theoretical question that is a real doozey for me, but maybe some of you could shed some light on it. This isn't a homework question or anything like that.

The question: So let's say that you have a 500 pound fly (yes, I mean the insect...I know you can't have 500 pound flies in real life) hovering in an airplane. Does the fly add to the total weight of the airplane when the insect is hovering? If it does, how? If it doesn't, why not?

For some reason, my physics professor couldn't answer this, and when my Dad asked this to his college professor in Physics 20-somewhat years ago, he couldn't answer it either.

EDIT: Please forgive me if this is actually an easy question...I'm not that great in physics. xD

 

[zeromodz]

If the fly does not touch the plane, it does not add to its mass.

 

[Vanadium 50]

Zeromodz is incorrect. The weight of the plane is exactly the same whether the fly is in flight or not. To hover, the fly has to push air downwards to the bottom of the plane. The force pushing downward by the air is exactly the same as the weight of the fly.

 

[Dr Lots-o'watts]

A 500 fly, flying within the aircraft, will make the (inside) air so turbulent the passengers will want to jump out, i.e. I agree with Vanadium50.

http://en.wikipedia.org/wiki/Wake_turbulence

 

[Fredrik]

A 500 fly, flying within the aircraft, will make the (inside) air so turbulent the passengers will want to jump out,

Yes, if there's a 500 pound fly on a plane, it's the air that will scare the crap out of the passengers.

 

[1MileCrash]

It doesn't add any mass to the plane...it adds weight to it. ;)

 

[JaredJames]

It doesn't add any mass to the plane...it adds weight to it. ;)

If the aircraft is traveling at a constant altitude, the wings are producing enough lift to counteract the total weight.

F = mg to put is very simply.

The total force required to be produced be the wings is equal to the weight of aircraft + weight of fly (fly is displacing air downwards with a force equal to its weight which is counteracted by the bottom of the aircraft).

F = m_aircraftg + m_flyg

F = (m_aircraft + m_fly)g

So it does add mass.

At least that's the way I'm seeing it.

 

[1MileCrash]

If the aircraft is traveling at a constant altitude, the wings are producing enough lift to counteract the total weight.

[...]

So it does add mass.

At least that's the way I'm seeing it.

I was only half joking, because the fly being in the plane doesn't add to the mass of the plane. I agree that the plane with the fly hovering in it is more massive than just the plane without the fly.

 

[starfish99]

500 pigeons in a truck

This reminds me of the old trick question in high school.A delivery man is delivering 500 pigeons in a truck and is approaching an old bridge. The weight of the truck+ the weight of the pigeons on a perch exceeds the weight limit of the old bridge. So the man startles the pigeons so they fly around while the truck safely crosses without collapsing the bridge.
Of course this won't work since they still add to the weight of the truck.
I also remember that if the pigeons were so startled that they all died of a heart attack at the same time and fell of their perches, for that split second of free fall before they hit the bottom of the truck, they would not add to the weight of the truck.

 

[Ivan Seeking]

This is a classic problem. Since the system is closed, as V50 stated, the weight doesn't change, but with one qualifier:

My first year in college, some of us tried to demonstrate this in the lab. Turns out that flies won't fly once put in a theoretical plane [aircraft].

 

[Danger]

As others have stated, it doesn't matter whether it's a 500lb fly or a million normal flies... the air that they displace downward while flying is equal to the weight that they would exert if merely perching. If that weren't the case, then sex in zero-g would be even more interesting than it already is.

 

[Lsos]

Mythbusters had an episode on this (a truck filled with pigeones). I don't remember what the result was, but I imagine it's in agreement with the general consensus :)

 

[zgozvrm]

... or say, you were standing on a scale and it read 180 lbs. A huge helicopter comes and hovers just a few feed directly over you. Would the weight shown on the scale increase? Would your weight increase?

 

[dnp33]

The question: So let's say that you have a 500 pound fly . Does the fly add to the total weight of the airplane when the insect is hovering? If it does, how? If it doesn't, why not?

First of all, you have to define "adding mass to the plane". If you mean does the fly add mass to the plane in terms of mass being accelerated:
if the fly were hovering on take off, the fly would not add mass, but it would add weight, due to the air being pushed down by its wings to lift itself up. Fly pushes on the plane, Plane pushes on the ground, fly goes up. It would not add mass because if the plane were to start to move, the fly would not start to move with it (other then the very small force the air would exert on the fly) unfortunately for the fly, it would very soon be hit by the back of the plane, at which point it would add mass to the plane. If, once again in mid flight the fly recovered, and started to fly again, it would be the same once again. The fly would not add mass, but it would add weight.

If you define the mass as the mass of the closed system of the plane, then the fly would always add mass to the plane.

 

[JaredJames]

If you define the mass as the mass of the closed system of the plane, then the fly would always add mass to the plane.

Just to clarify, this is what I was referring to in my above post. The mass of the entire system.

However given the fly is detached from the aircraft, would I be right in saying the only way to increase the overall aircraft weight is to increase the mass or the gravity (or acceleration). If gravity is considered constant (aircraft flying straight and level), then it must be the mass that increases.

So the mass does increase either way?

 

[dnp33]

if you think about it in terms of Newtons law F=ma then it makes sense.
to accelerate the entire system by, say, 20m/s^2 then the force required is F=(m-plane+m-people+m-fly +...)*20m/s^2.

 

[JaredJames]

Yeah, but what I'm saying is that yes, it does increase the weight whilst hovering, but it also increases the mass.

(All of this is whilst flying at constant speed / altitude)

I might be missing something here, but if the acceleration is equal and constant on the fly and aircraft then as per my equation above it must be the mass that changes.

I think I've confused myself here.

 

[zgozvrm]

I'd say the fly adds weight as long as it's downward thrust of air is pushing on the plane. That means whether the fly is inside the plane or outside and directly above it, the added force of the downward flow of air causes the weight of the plane to increase.

So, the question now becomes this:
If the effect of the downward force of air is the same whether the fly is inside or outside of the plane, then is the fly adding to the mass of the plane in one or both of those situations (or neither)?

I tend to think neither.

I guess it depends on whether the air inside the plane is considered to be part of the mass of the plane. (Which may be so, if we're talking about a pressurized cabin).

 

[dnp33]

The OP stated that he means the mass of the system, which means everything inside the plane. The Planes mass itself cannot change.
I think the problem here is that people Aren't distinguishing between mass and weight. If you look at the system from the outside, the force required to hold up the plane is equal to the acceleration of gravity times the mass of everything in the plane, fly included.

 

[JaredJames]

I think the problem here is that people Aren't distinguishing between mass and weight. If you look at the system from the outside, the force required to hold up the plane is equal to the acceleration of gravity times the mass of everything in the plane, fly included.

That's what I'm looking at, so I'm taking my answer as correct. The mass of the system does increase.

 

[Danger]

Something just came to mind which, for some unknown reason, I never thought of before.
Say that the bug or bird or whatever stops flying and drops to the floor. During that brief free-fall, would the apparent weight of the system change? I'm thinking no, because the column of air being compressed beneath it would be the same as the flying mode, but I'm not sure.

 

[dnp33]

I'm thinking no, because the column of air being compressed beneath it would be the same as the flying mode, but I'm not sure.

If the colum of air was the same, the bird would hover without falling.

 

[Vanadium 50]

During the brief free fall the center of mass of the airplane-fly system is accelerating. You need to take that into account.

 

[Danger]

If the colum of air was the same, the bird would hover without falling.

Due to my poor phrasing, your response is correct. I was actually thinking more of the way that a vented airbag (for stuntmen) works.
Vanadium, thanks for input. I never thought of that.

 

[dnp33]

I was actually thinking more of the way that a vented airbag (for stuntmen) works.

I'm not sure exactly what you mean could you elaborate I am curious now.

 

[DaveC426913]

Something just came to mind which, for some unknown reason, I never thought of before.
Say that the bug or bird or whatever stops flying and drops to the floor. During that brief free-fall, would the apparent weight of the system change? I'm thinking no, because the column of air being compressed beneath it would be the same as the flying mode, but I'm not sure.

As the fly fell to the floor, the plane woud rise. If the plane outweighs the fly by 10x, then the fly will fall ~91% of the distance while the plane will rise ~9.1% of the distance.

It is simply a centre of mass issue.

Replace the column of air with a more intuitive, rigid piston, and it will make sense.

Regardless of how slow the fly falls to the floor because of escaping air, the plane will take that same time and same distance to rise. If the fly took five minutes to settle to the floor, the plane would take 5 minutes to rise 9.1% of the way.

 

[GT1]

As the fly fell to the floor, the plane woud rise. If the plane outweighs the fly by 10x, then the fly will fall ~91% of the distance while the plane will rise ~9.1% of the distance.

It is simply a centre of mass issue.

Replace the column of air with a more intuitive, rigid piston, and it will make sense.

Regardless of how slow the fly falls to the floor because of escaping air, the plane will take that same time and same distance to rise. If the fly took five minutes to settle to the floor, the plane would take 5 minutes to rise 9.1% of the way.

I don't understand why the plain needs to rise, what force cause it to raise and how does this force created?

 

[JaredJames]

When in level flight (constant altitude) the wings are producing an amount of lift equal to the weight of the aircraft + the force exerted on the bottom of the aircraft by the fly.

If the fly stops exerting the force on the aircraft (or reduces the magnitude of force) then the lift from the wings is now greater than the weight of the aircraft + force from fly. So the aircraft rises.

If the fly exerted more force, the aircraft would sink.

 

[DaveC426913]

Yes. If I were holding a 500lb. anvil 6ft above the floor, I would weigh 700lbs. If I then dropped it, I would weigh 200lbs. for a 1/2 second. Therefore the plane is lighter for 1/2 second.

Note that I could completely sidestep the whole air column thing by placing the anvil in a vacuum chamber, held by a string. The 500lb. weight is born by me, standing outside the chamber, on the plane's deck.

 

[zgozvrm]

I'm not sure if this is correct, but consider this:

Suppose a 500 lb man jumped straight up inside the plane. The plane is moving at 500 mph. The man lands exactly where he jumped from (the plane doesn't move out from under him). This is because the air inside the plane is part of the mass of the plane, as is the man (whether he is standing, or in the air). If the man were NOT part of the mass of the plane while he is not on the floor of the plane, then the plane would have moved out from under him.