[simeonie (OP)]

I was thinking.

If you got a really big lorry full of birds that were all sat down and weighed it. Would the weight be different if all the birds in the lorry flew in the air inside of the lorry,. You know what I mean?

Any ideas people?

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[rosy]

I'd *guess* that if the lorry were sealed so air couldn't get in or out (you can have an oxygen supply if you want to stop them dying, I'm interested in pressure differences!) then the weight would measure the same... the birds are, after all, pushing air down onto the floor of the lorry.
But I haven't thought about it very hard..

[johndiver]

I recall my physics prof asking us the same question, and we decided the weight would AVERAGE to be the same once the birds were flying inside the box. The reasoning goes that the birds are gliding through the medium, which is part of the mass in question. When the birds flap their wings, they are creating pressure waves which are seen as fluctuations in the weight of the box.
Here's an analogy for humans: our blood. As the heart pumps the blood upward into the aorta, our body weight appears to increase momentarily, as per Newton's action/reaction law. A sensitive weighing scale can catch this fluctuation. Now if one wants to be anally precise when giving their body weight, they can say "165 pounds +/- 3 ounces".
I recall a spiritual movement that was based on this weight fluctuation, although they neglected to tell their followers the real reason for it. Instead they claimed that a dying person placed on a weighing scale will lose a few ounces of weight at the moment of death. This, they claimed, was the weight of the soul as it left the body. Ergo, souls have mass and ergo they exist!

[simeonie (OP)]

so it wouldnt weigh exactly the same but will still weigh more that an empty box or lorry because of the pressure from the birds flapping their wings.........?

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[Solvay_1927]

I remember seeing this topic discussed (to death) on another site, so I've searched it out - it can be found on:

http://www.physlink.com/Community/Forums/viewmessages.cfm?Forum=10&Topic=2351

P.S. I wouldn't normally recommend the physlink discussion forum - I generally find it humourless and dull (due to the anal way in which the moderators run it) - but the discussions on there can occasionally be of use.


"Eagles may soar, but weasels don't get sucked into jet engines."

[A Big Mug]

quote:Originally posted by simeonie

so it wouldnt weigh exactly the same but will still weigh more that an empty box or lorry because of the pressure from the birds flapping their wings.........?

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No, it would weigh exactly the same.  The weight of the birds does not go away.  Like one of the previous folks said, the weight of the bird is supported by a column of air that exends down to the floor of the truck.  Now if the birds were migrating African Swallows carrying coconuts that would be different.

[ukmicky]

But wouldnt it be dependent on whether the box was air tight or not. As the bird flaps its wings its weight is transfered into a force,  air presure .  which would escape through any air gaps and change from an internal force and become an external force , not part of the system (if you get what i mean ). any energy treansfered to the  air  that was able to leave the box through the forces of the bird flapping its wings would mean less weight in the box.

but in the end its a lorry which is carrying birds so it can't be air tight, so forces would be lost to the outside enviroment,in other words the lorry would weigh less if the birds were flying.[]


ps Come to think of it even if it was a sealed box, it would still slowly lose weight.
As the birds expend energy flapping their wings they would lose weight as heat which would slowly migrate outside through the sides of the lorry. so the box would always weigh less if they were flying. []
Michael                                      

[neilep]

...Surely the weight of the birds is supported by themselves !..they are flapping their wings and supporting their own weight.


Men are the same as women.... just inside out !!

[ukmicky]

They maybe using there own energy, but there using the column of air underneath them to hold them up. And if that column of air doesn’t transfer its entire extra load to the lorry as downward air pressure caused by the bird bringing its wing down because it’s partly escaping outside the lorry then there's less weight bearing down on the lorry.

If the lorry is sealed then Newton’s 3rd law comes into play which States that for every action (force) there is an equal and opposite reaction


But if the lorry is not sealed then the bird  transfers all its flapping energy to the air but the air can’t transfer its acquired energy (downward force) to the lorry because some of it is being lost outside the lorry
.


Michael                                      

[neilep]

..so, If I a ten ton canary was hovering seven feet off the ground inside a totally sealed lorry and I stood underneath it....am I to understand that I would be crushed ? even though the canary is hovering above me ?

I am trying to understand and appreciate the explanations....perhaps i should check out some of these laws.

Men are the same as women.... just inside out !!

[ukmicky]

yeah basically your ten ton yellow canary would have to produce ten tons of lift,so would produce ten tons worth of down draft, but the box it was in would have to very large because it wouldnt fit in your average cage(well not mine any way)
so its force would be dissapated over a large area. but it would still blow your wig off.

oooh sorry didnt mean to tell everyone about your..w cough cough... nearly said it again sorry.[]
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Newtons laws,have a read their easy to understand + plus its good to know about internal and external forces ask your wife[]

Michael                                      

[neilep]

Newtons laws.....easier than Quantum I hope ! I asked wifey and she showed me the special apparatus she uses to apply these laws ![]...and in addition to the wig...I'm more concerned about my false arm, leg and bum being blown off !![]

Men are the same as women.... just inside out !!