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Q: Weight of a fly in an airplane ( No Answer,   10 Comments )
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 Subject: Weight of a fly in an airplane Category: Science Asked by: tulsavp-ga List Price: \$2.00 Posted: 24 Dec 2005 15:21 PST Expires: 23 Jan 2006 15:21 PST Question ID: 609552
 ```In the movie Lindberg with James Stewart, a fly rides along in the cockpit of the plane. Does the planes weight change (by the weight of the fly) depending on whether the fly is flying around within the cockpit or if it has landed on something within the plane? Why?```
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 Subject: Re: Weight of a fly in an airplane From: pinkfreud-ga on 24 Dec 2005 15:31 PST
 ```From a discussion of a similar problem, involving a pigeon rather than a fly: "The pigeon takes off and flies around the cabin. What does the plane weigh? Exactly the same. On average, the pigeon must exert one pound of downward force on the cabin air to keep itself aloft, and the cabin air in turn presses down on the airplane." http://www.straightdope.com/classics/a3_144.html```
 Subject: Re: Weight of a fly in an airplane From: antipodean_manc-ga on 05 Jan 2006 19:31 PST
 ```I believe the comment made by pinkfreud is partly correct and partly incorrect. It is correct becuase it is the right answer but for the wrong wrong reason. The answer should not make reference to cabin air pressure as this has no impact on the weight of the plane (or its mass). The question is slightly misleading as it refers to weight. If the plane is flying level then it is weightless. The upward force of the wings is matched exactly by the downward acceleation of the aircraft due to gravity. It has no weight. If the fly takes of, flys and land in the cockpit, the fly is only changing the internal forces on the fly within the cockpit. It has no effect on the external forces of the aircraft as the cockpit is sealed. Perhaps the question should consider the mass of the air plane rather than weight. The mass stays constant no matter what the airplane is doing or what the fly is doing in the cockpit. The mass is the sum of the mass of plane and its contents including the fly and the air surrounding the fly. It matters not whether the fly is crawling on the cabin floor or is flying in the air, the mass of the plane includes the fly and the air around the fly. By taking off, the fly only adjusts the internal air pressure not the mass of the air. One last consideration. If the fly suddenly materialised [impossible but hypothethetical) in mid air in the plane, the plane mass would increase by the mass of the fly. Why? The fly displaces the air around it creating less air volume but more pressurised air - the mass of the air would remain identical. The weight then changes only by the fly not by the air around it.```
 Subject: Re: Weight of a fly in an airplane From: joephysics-ga on 07 Jan 2006 21:37 PST
 `pinkfreud is correct. The other person's answer makes no sense.`
 Subject: Re: Weight of a fly in an airplane From: antipodean_manc-ga on 08 Jan 2006 19:49 PST
 ```Perhaps you could elaborate which bit of my answer you believe not make any sense. To make it easier, I will repeat all my statements in simple for, then you can pick and choose: 1) Pinkfreud has the correct answer 2) Pinkfreuds explanation for that answer is incorrect 3) The question is better expressed as mass rather than weight 4) The plane is weightless when flying in equilibrium 5) The fly (in a sealed cockpit) can only affect the internal air pressures within the cockpit or the stresses on the aircraft body 6) The fly (In a sealed cockpit) can not influence the upwards or downwards forces on the airplane itself 7) The existance of the fly effects the mass of the aircraft. The mass remains constant whether the fly is crawling or flying in the cockpit. If the cockpit is NOT sealed, the fly will certainly effect the weight of the aircraft by creating repulsion forces by the air being expeled from the aircraft under pressure as the fly beats its wings. I leave you with a question to illustrate my point. If a man in a sail boat blows into the sail with a pair of bellows, does the boat move? What if he pushes against the mast, does the boat move? In both cases the answer is no, all he does is creates stress in the structure of the boat, but no force moving the boat in any direction. This is the same effect of tthe fly beating its wings in the sealed cockpit, it can not effect the forces on the aircraft with repect to the outside world (hence, no effect on the weight). Are you happy with the distinction between mass and weight?```
 Subject: Re: Weight of a fly in an airplane From: antipodean_manc-ga on 09 Jan 2006 13:49 PST
 ```OK, I withdraw my previous comment. I have thought about it a little more and it turns out I am an arse. Pinkfreud-ga is correct.```
 Subject: Re: Weight of a fly in an airplane From: sorwin-ga on 09 Jan 2006 15:38 PST
 ```A complete answer must take air pressure into account. Anything which flies must support itself by generating a pressure difference between its lower and upper surfaces. Hence the air pressure below a flying object is greater than that above it. It follows that the *average* air pressure (extending right down to the floor) below the flying object is greater than the average pressure above it. Converting pressure into force withing a sealed container, it follows that the consequent downward force acting on the bottom of the container is greater that the upward force acting on the top of the container. As an example of this, if I lie flat on the ground and a hovercraft passes over me, I experience an increase in the pressue of air. It's the same effect when a fly buzzes overhead inside an aircraft, or anywhere-else.```
 Subject: Re: Weight of a fly in an airplane From: egon_spangler-ga on 11 Jan 2006 08:19 PST
 ```Indeed air pressure plays a roll. As the fly takes off the plane actuialy would "weigh" slightly more becuase the energy the fly is using to push itself up is also pushing the plane down an equal amount. (If the fly were in flight like an airplane or a bird flies then it would be lift, not downward thrust, that kept the fly aloft) That would have a different effect.```
 Subject: Re: Weight of a fly in an airplane From: egon_spangler-ga on 11 Jan 2006 08:21 PST
 ```Sorry... i missed a few sentences in my answer. "Because the fly is converting potential energy of chemecals in it's body into kenetic energy of creating downforce with it's wings, it will push the plane down slightly."```
 Subject: Re: Weight of a fly in an airplane From: beggarslice-ga on 25 Jan 2006 15:33 PST
 ```Perhaps the answer only involves the difference between the weight of the fly and the weight of the volume of air displaced by the volume of the fly.```
 Subject: Re: Weight of a fly in an airplane From: jetter-ga on 09 Mar 2006 17:35 PST
 ```The gedanken experiment posed by antipodean_manc regarding the man in a sail boat is interesting. However, the assertion that the boat would not move if the man blows into the sail with a bellows is incorrect. The boat would move, albeit very inefficiently. (Note that sail boats do not move because the wind fully inflates a sail, but rather the sail acts as an airfoil and lift is generated in the horizontal plane. This lift is what propels the sail boat through the water.) The bellows would push air into the sail. Pushing the air into the sail exerts a pressure on the sail, and therefore a small force, since the sail has a surface area. Thus, the force would push a little bit on the sail, and the boat would move a little bit. This is no different that if a fan were used to push air into the sail. It would act the same way and the boat would move. If a force is produced, by a man with a bellows or an engine, then there must be an opposite reaction to that force.```