Weighty Problem

[Pangolin]

A lorry is carrying a large cage full of birds which are all standing on the floor of the cage. If the birds decide to fly about in the cage will the weight of the lorry and its load change?

Answer please and explanation if possible, thank you.

[factor-fiction]

Yes, if a 5kg albatross flew just above my cage I doubt that the reading on the scales would go up by 5kg.

Regarding the general birds in a cage/ birds in a lorry problem, I wonder whether it depends on whether they are fully enclosed- eg does it depend on whether the bird is in a mesh cage or in a solid box, and does it make any difference whether the lorry is sealed or is an open trailer?

[Andyvon]

Yes, I'm sure the lorry would get lighter.

Suppose eight people are inside a lift in a lift shaft. If they all jumped up at the same time the weight of the lift would change to the same as when it was empty until they all landed on the floor again. It would be the same if the birds left the floor of the lorry at the same time.

[factor-fiction]

Is that right Andyvon? I'd have thought the total weight of the lift and the contents would stay the same

[vascop]

Factor
You can't have it both ways! This is analogous to the birds situation apart from the flapping of wings and the fact that the lift is moving.
Andyvon: Why does it have to be in a lift? What about people standing on a weighing station and jumping?

[factor-fiction]

I'm not having it both ways, I don't think, vascop.
In an enclosed lorry or lift I think the weight stays the same whether the people inside are standing, lying down or flying.
I think I'm just saying it gets less straightforward when it's not an enclosed space, although I'm not sure why

[vinginge]

I'd say the wieght would change.
The analogy is if you stand on the bathroom scales and then jump into the air. Momentarily, the scales will show a heavier weight measure due to the sudden down force on the scales but after that the measure will revert to zero until you land back on the scales.

[Andyvon]

A weighing station is a far better analogy Factor. I wish I'd thought of that instead.

Vinginge's example of the bathroom scales says the same thing. The people would be weightless for a few moments until gravity returned them to the scales so the scales would register zero for those moments.

[WyeDyed]

The important point is that it is a closed system. If the birds are in a box, the weight remains the same; as soon as you open the box, you no longer have a closed system and the weight would alter.

[factor-fiction]

Exactly. The key here is that you need an enclosed system. Obviously if a massive bird weighting 20kg is sitting on a weighing station and then flies up in the air and hovers a few hundred feet above the scales then the weight reading will fall to zero. But if the bird was in a big sealed box which was on the weighing station then it seems to me the box and contents would stay the same whether the bird was standing on the floor, hanging from the ceileing or flying around. That's just an intuitive answer though-I can't prove it and would need to research it to confirm and explain it

[Andyvon]

Factor -

I think the moment the bird lost contact with the box the weight of the box would change until the bird settled in contact with the box again. Although the bird has the same mass, it doesn't exert the same downward force through gravity while it's airborne.

Isn't it the same with astronauts in an orbiting spacecraft? The spacecraft has a certain mass while the astronauts are secured in their seats but the mass changes when the astronauts are floating and weightless. They then have their own mass independent of the spacecraft.

[factor-fiction]

I don't know. It just feels wrong that the mass of a closed object can cahnge depending on where the interior parts are. Does that mean the mass of the earth changes as people jump up and down, aeroplanes fly/land and birds decide to fly off the trees and rain lands on the ground?

[vascop]

Factor
Careful not to mix up mass and weight.

[factor-fiction]

I know there is a difference although in every day use I can't see the difference. I'm not sure what difference it makes to this question.

[vascop]

wyeyed
You cannot be serious!

[vascop]

Factor
The mass of the cage plus birds is the same whether the birds are perching or flying, but the weight will change depending on what the birds are doing.

[factor-fiction]

Thanks, but could you explain that as I'm a bit rusty on it. I thought weight was mass x g (accelaration due to gravity). As mass is unchanged are you saying that it is g has become lower or zero for the bird?

[vascop]

Let's say the bird is hovering. Its weight is then being cancelled by an upward force produced by its wings. So the bird does not contribute to the overall weight when hovering, but it's still inside the closed cage and so the total mass is the same.

[vascop]

or if someone on a weighbridge jumps into the air, then the mass of person plus weighbridge is constant but the weight measured by the weighbridge would change.

[factor-fiction]

Thanks. So does that mean the weight of the earth will fluctuate very slightly depending on things like rain and birds/aeroplanes flying/landing?

[gen2]

Every action has an equal and opposite reaction.

Whatever force is keeping the bird up will have an equal and opposite force down on the floor of the box just as if it were standing there. In this example, that force is an increase in air-pressure upon the floor.