# Something else to think about

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There is a problem i have never managed to find a definite solution to. It goes as follows:A pressurised aircraft (thus a closed system) carries a flock of birds. Suppose that the birds are flying inside the aircraft. Will this make the aircraft lighter?My own suggestion is no, based on simple conservation of momentum in a closed system. The birds flapping their wings will eventually exert a downward force on the aircraft body. Still, I have seen all kinds of resoning that supports the opposite conclusion.-

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>Will this make the aircraft lighter?No, and it actually can be neatly proven by using the conservation of energy - imagine the aircraft climbs therefore both the aircraft and birds accumulate additional potential energy. If the aircraft was lighter with birds flying you would manage to gain potential energy (for birds) without doing any work ...Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

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Well, that IS neat...-

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Technically the birds would not gain potential energy in a pressurised cabin. They would still be the same distance above the cabin floor. They would not gain potential energy unless the pilot executes a negative G pushover.When the birds land; of course the plane will be heavier!!! Haven't you heard the joke about the trucker hitting his rig full of live chickens with a crowbar just before the weigh station???FLYing? It's cool. Trillions of birds and insects can't be wrong.

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I think he meant in relation to the aircraft.

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1) Assuming the cabin air pressure is the same as the air pressure where the aircraft was weighed.2) Assuming a steady state, straight and level condition.The air in the aircraft has to support their weight, the aircraft has to support its cabin air, so the aircraft is the same weight.Think of it this way. If you fill a tank (5kg) with water (10kg) and add a boat that weighs 1kg, the whole lot will weigh 16kg not 15jg. It doesn't matter if the boat springs a leak and sinks or floats or partially floats.

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In this context, potential energy is in relation to earth because it's based on the earth's gravitation field. Potential Energy = Mass * Acceleration due to gravity * Height.

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They are still the same height AFL (above floor level). They can not trade that energy- a fundemental difference. The pilot can "trade" for them though.Best Regards; Donny:-waveFLYing? It's cool. Trillions of birds and insects can't be wrong.

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The purpose of introducing the potential energy was to use conservation of energy to show how birds add to the weight. If you don't understand who this mental 'experiment' works then you are stuck with the 'floor level'. Sorry, I wish I had time to write the whole thing - I wrote it (too) briefly for those who are well versed in physics.Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

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