# Birds in a van

I came across this question while looking around the forums at physlink.com:
There are birds of mass 20 tonnes in a van of mass 10 tonnes. The back of the van is closed(no air can go in or come out). If now the birds start flying, will the weight of the van decrease?

I ask this question here because the folks at physlink at quite divided on this topic. My opinion is that the weight will not decrease since the bird will apply a force on the air to fly. The air will apply the same force on the floor of the van(since it can't go out). Hence the weight shouldn't decrease.
Another analogy is that the birds are acting as point particles. They will interact with other birds and with air particles(like another gas mixed in air). So the weight won't decrease.

Another question:
Can a bird in a closed container lift the container up? This is surely possible if the bird is in a cage, but probably not in this case.

For the first one, I'm really divided in my opinion, so I don't know.

ssj5harsh said:
Another question:
Can a bird in a closed container lift the container up? This is surely possible if the bird is in a cage, but probably not in this case.

A cage isnt a closed container, so it could lift a cage. If it was in a balloon or something, then I dont think it would be able to lift itself up. It wouldnt have enough airflow to work with to generate its lift.

ssj5harsh said:
I ask this question here because the folks at physlink at quite divided on this topic. My opinion is that the weight will not decrease since the bird will apply a force on the air to fly. The air will apply the same force on the floor of the van(since it can't go out). Hence the weight shouldn't decrease.
Another analogy is that the birds are acting as point particles. They will interact with other birds and with air particles(like another gas mixed in air). So the weight won't decrease.

wouldn't the bird end up putting pressure on the sides aswell, and therefore , from outside, the van will lift slightly, but it will also rock as the birds put pressure on the sides...

When the cage is closed and the birds start flying:

A flying bird inside the cage will try to support its own weight , therefore the thrust with which the bird swaps its wings will equal the mg downwards bu the force with which air molecules strike the surface with the same thrust , thus the whole weight still remains the same.You can say the basic interaction remains the same.

ssj5harsh said:
I ask this question here because the folks at physlink at quite divided on this topic.

Evidently the folks at physlink have forgotten that momentum is conserved.

If the birds could make the van lighter, then with no gravity they could accelerate it with no outside force acting on it. That's impossible. So making it lighter is too.

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SGT
I agree with you. The force needed to make the bird fly is provided by the air, wich will exert the same force on the floor.
As hexhunter said, the movement of the air will be turbulent and thus exert pressure in the sides and even the ceiling of the van, but all those forces cancel out. The force in the floor will be the weight of the birds plus the force on the ceiling.
Of course a bird can't lift a closed container anymore than you can lift yourself by pulling at your hair, like did the baron of Münchausen.

Doc Al
Mentor
If the birds start out sitting on the floor of the van, then begin flying around, realize that the center of mass of the system has changed. During that transistion, the apparent weight of the van would fluctuate.

hexhunter said:
wouldn't the bird end up putting pressure on the sides aswell, and therefore , from outside, the van will lift slightly, but it will also rock as the birds put pressure on the sides...

Wouldn't the molecules of a gas also exert a similar force causing a closed container which is sitting on the table to rock slightly? I think the pressure on either side cancels the force out.
This would also be true for a large number of birds in said van.

doc al said:
If the birds start out sitting on the floor of the van, then begin flying around, realize that the center of mass of the system has changed. During that transistion, the apparent weight of the van would fluctuate.

Is there any correlation between centre of mass and weight?
W=mg
The change of the position would only cause a small(perhaps negligible) change in 'g'. As far as I know centre of mass only related to stability.

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Doc Al
Mentor
ssj5harsh said:
Is there any correlation between centre of mass and weight?
W=mg
The change of the position would only cause a small(perhaps negligible) change in 'g'. As far as I know centre of mass only related to stability.
The question really isn't asking about the actual weight of the van (the force of the earth's gravitational pull, w = mg), but about the apparent weight that a scale would register. (Imagine the entire van on top of a giant, very accurate scale.) The apparent weight is equal to the normal force of the supporting surface.

Now rethink your answer. Does the force that the ground (or scale) exerts on the van change at any time as the birds begin to fly?

Here's an exaggerated version of the same thing that may be easier to visualize. Imagine yourself sealed up in a large wooden crate. The crate rests on top of a scale. You jump up and down. Will the scale reading change?

I get your point now Doc Al. Thanks.

SGT
Doc Al said:
The question really isn't asking about the actual weight of the van (the force of the earth's gravitational pull, w = mg), but about the apparent weight that a scale would register. (Imagine the entire van on top of a giant, very accurate scale.) The apparent weight is equal to the normal force of the supporting surface.

Now rethink your answer. Does the force that the ground (or scale) exerts on the van change at any time as the birds begin to fly?

Here's an exaggerated version of the same thing that may be easier to visualize. Imagine yourself sealed up in a large wooden crate. The crate rests on top of a scale. You jump up and down. Will the scale reading change?
Yes, in order to jump you must accelerate your mass, so in the instant of the jump the normal force will be greater. But if after jumping you could fly the normal force would be only your weight. The same is true for the birds. In the moment they start to fly the weight is greater, but it stabilizes afterwards.
Anyway I don't see what does this relate to the displacement of the center of gravity.

Doc Al
Mentor
SGT said:
Yes, in order to jump you must accelerate your mass, so in the instant of the jump the normal force will be greater. But if after jumping you could fly the normal force would be only your weight. The same is true for the birds. In the moment they start to fly the weight is greater, but it stabilizes afterwards.
Right.
Anyway I don't see what does this relate to the displacement of the center of gravity.
The movement of the center of mass implies an acceleration, thus an increased normal force. (Just another way of looking at the same thing.)

SGT
Doc Al said:
Right.

The movement of the center of mass implies an acceleration, thus an increased normal force. (Just another way of looking at the same thing.)
OK! I didn't realize that in your previous post you mentioned that the weight variation occurred during transition. I agree with you.