ChatterBank0 min ago
Weighty Problem
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.
Answer please and explanation if possible, thank you.
Answers
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Factor30, It is rather meaningless to speak of the 'weight' of the earth unless you mean in respect to the sun's gravity. The weight of the earth would then be its mass multiplied by the value of solar gravity. Since the mass of the earth/atmosphere system remains constant, its weight will remain constant.
gen2
That's a simplified version of what would happen in a real situation because it assumes that the air behaves like a rigid solid object, which it doesn't because it compresses and causes a wave which dissipates over distance. If the cage were very large then the effect would dissipate and the effect on the base would be negligible. The energy in the wave would be converted to sound or heat etc.
That's a simplified version of what would happen in a real situation because it assumes that the air behaves like a rigid solid object, which it doesn't because it compresses and causes a wave which dissipates over distance. If the cage were very large then the effect would dissipate and the effect on the base would be negligible. The energy in the wave would be converted to sound or heat etc.
Thanks gen2. I wonder whether the big increase in the human population has affected the mass of the earth- or is it cancelled out by the consumption of food/water resources
Using the weighbridge example, are we saying that if a lorry goes over a weighbridge the driver and his mate could keep the weight down slightly by juggling heavy objects/throwing them to each other as the lorry goes over the scales? Or could some contents be tied to inflated helium balloons so they float?
Using the weighbridge example, are we saying that if a lorry goes over a weighbridge the driver and his mate could keep the weight down slightly by juggling heavy objects/throwing them to each other as the lorry goes over the scales? Or could some contents be tied to inflated helium balloons so they float?
vascop, the force per unit area would be lower for a larger cage, but the total force would be the same (for a closed system)
Yes factor30, they would cancel out.
Re. the juggling: just like the birds, the force needed to throw the object in the air would exert an equal force downwards.
Re. the helium: Yes the lorry would be lighter since some heavier air has been displaced to make room for the lighter helium.
Yes factor30, they would cancel out.
Re. the juggling: just like the birds, the force needed to throw the object in the air would exert an equal force downwards.
Re. the helium: Yes the lorry would be lighter since some heavier air has been displaced to make room for the lighter helium.
Thanks gen2- my question fabout the lorry, juggling items and using helium balloons (to lift up heavier objects rather than just for helium to replace heavier gasses) was really for vascop. I'm not totally convinced either way but if vascop is right then it would seem there are techniques that could be used to reduce the apparent weight of a container
factor30, Helium is only able to lift weights BECAUSE it displaces heavier air.
Every 22.4 litres of helium weighing 4g will displace 22.4 litres of air weighing just over 28g and thus will be able to lift 24g (including whatever is containing the helium.
Every 22.4 litres of helium will therefore make the lorry 24g lighter. It doesn't matter whether the helium is actually lifting something, as long as it is in the lorry.
Every 22.4 litres of helium weighing 4g will displace 22.4 litres of air weighing just over 28g and thus will be able to lift 24g (including whatever is containing the helium.
Every 22.4 litres of helium will therefore make the lorry 24g lighter. It doesn't matter whether the helium is actually lifting something, as long as it is in the lorry.
I've been reading all the contributions to this discussion and I think everything hinges on what we mean by a closed system. I was thinking initially that contributors to this post were calling a cage a closed system as long as the birds stayed in the cage.
In physics a closed system is defined as one which does not exchange matter with its surroundings, but can exchange energy with its surroundings. An isolated system is one which does not exchange matter and/or energy with its surroundings Now a cage with birds in it is by these definitions neither a closed system nor an isolated system since it can exchange matter with its surroundings in the form of air, dust etc and can also exchange energy in the form of heat and so on.
It is clear that in nature it is impossible to have a truly closed system or an isolated system.
If, as some people have said, the flapping of the birds' wings keeps them in the air and that there is an equal and opposite reaction on the bottom of the cage then for this to work the cage would have to be a sealed unit so that no air could escape (as it could with a normal cage). But the cage must also be an isolated system as well so that the sound waves caused by the birds flapping their wings would not cause the walls of the cage to vibrate, which would be an exchange of energy to the surroundings.
So to sum up I now think that I agree that if the system is closed and isolated according to the definitions above then when the birds fly the weight of the closed system will not change.
But of course this is very difficult to arrange in practise.
Gen2: But you are bringing in the Helium from outside so that's cheating. You would have to add the weight of the Helium and so the total weight would be the same.
In physics a closed system is defined as one which does not exchange matter with its surroundings, but can exchange energy with its surroundings. An isolated system is one which does not exchange matter and/or energy with its surroundings Now a cage with birds in it is by these definitions neither a closed system nor an isolated system since it can exchange matter with its surroundings in the form of air, dust etc and can also exchange energy in the form of heat and so on.
It is clear that in nature it is impossible to have a truly closed system or an isolated system.
If, as some people have said, the flapping of the birds' wings keeps them in the air and that there is an equal and opposite reaction on the bottom of the cage then for this to work the cage would have to be a sealed unit so that no air could escape (as it could with a normal cage). But the cage must also be an isolated system as well so that the sound waves caused by the birds flapping their wings would not cause the walls of the cage to vibrate, which would be an exchange of energy to the surroundings.
So to sum up I now think that I agree that if the system is closed and isolated according to the definitions above then when the birds fly the weight of the closed system will not change.
But of course this is very difficult to arrange in practise.
Gen2: But you are bringing in the Helium from outside so that's cheating. You would have to add the weight of the Helium and so the total weight would be the same.
/// But you are bringing in the Helium from outside so that's cheating. You would have to add the weight of the Helium and so the total weight would be the same. ///
It was in fact factor30 who introduced the helium as an added complication.
For that new scenario, I already calculated in the weight of the helium (4g per 22.4 litres at STP) and subtracted the weight of the displaced air (28g per 22.4 litres at STP for nitrogen). There would indeed be a small reduction in overall weight of the lorry.
Apart from that, you have summed it up well vascop.
It was in fact factor30 who introduced the helium as an added complication.
For that new scenario, I already calculated in the weight of the helium (4g per 22.4 litres at STP) and subtracted the weight of the displaced air (28g per 22.4 litres at STP for nitrogen). There would indeed be a small reduction in overall weight of the lorry.
Apart from that, you have summed it up well vascop.
-- answer removed --
Wow this moved on since I last looked at it.
No your 5kg albaltross would probably not show all it's 5kg on your scales since the air pressure would not all hit your scales, it will be dispersed around it, so you are not considering a closed system.
Yes you may jump up and down on the weighng station and the momentary weight will vary but the average weight remains the same. When you started the jump the weight went momentarily up, then when you were at the peak of your jump it was down, and the fluctations cancel out over a short time.
No your 5kg albaltross would probably not show all it's 5kg on your scales since the air pressure would not all hit your scales, it will be dispersed around it, so you are not considering a closed system.
Yes you may jump up and down on the weighng station and the momentary weight will vary but the average weight remains the same. When you started the jump the weight went momentarily up, then when you were at the peak of your jump it was down, and the fluctations cancel out over a short time.
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