Which weighs more in atmosphere, $1,{rm kg}$ of steel or $1,{rm kg}$ of feathers?
I'm having a discussion at the moment regarding the mass of $1,{rm kg}$ of feathers and $1,{rm kg}$ of steel.
The person I'm arguing with states that $1,{rm kg}$ of feathers will be lighter when weighed, compared to the $1,{rm kg}$ of steel, because the feathers are more buoyant.
She has done her calculations for the density of feathers and works out that $1,{rm kg}$ of feathers will displace $400,{rm L}$ of water. And because the feathers are more buoyant than the steel they would actually weigh less.
We are talking about the kilogram as a unit of mass, not weight.
The argument goes that if a scale is balanced perfectly in vacuum, it is not in air. The feathers, being more buoyant in air, would cause the scale to tip toward the steel.
I’m sure she is wrong, and even though buoyancy may be a factor, she calculates that the feathers would only have about half the weight in air that they do in vacuum.
So the question is, if equal masses of feathers and steel were balanced in a vacuum, would they still balance in $1,{rm atm}$? If not, what would be the difference in weight?
mass density fluid-statics buoyancy weight
edited Dec 20 at 8:11
Qmechanic♦
101k121831142
asked Dec 20 at 0:29
James Thorpe
865
New contributor
James Thorpe is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
add a comment |
I'm having a discussion at the moment regarding the mass of $1,{rm kg}$ of feathers and $1,{rm kg}$ of steel.
The person I'm arguing with states that $1,{rm kg}$ of feathers will be lighter when weighed, compared to the $1,{rm kg}$ of steel, because the feathers are more buoyant.
She has done her calculations for the density of feathers and works out that $1,{rm kg}$ of feathers will displace $400,{rm L}$ of water. And because the feathers are more buoyant than the steel they would actually weigh less.
We are talking about the kilogram as a unit of mass, not weight.
The argument goes that if a scale is balanced perfectly in vacuum, it is not in air. The feathers, being more buoyant in air, would cause the scale to tip toward the steel.
I’m sure she is wrong, and even though buoyancy may be a factor, she calculates that the feathers would only have about half the weight in air that they do in vacuum.
So the question is, if equal masses of feathers and steel were balanced in a vacuum, would they still balance in $1,{rm atm}$? If not, what would be the difference in weight?
mass density fluid-statics buoyancy weight
edited Dec 20 at 8:11
Qmechanic♦
101k121831142
asked Dec 20 at 0:29
James Thorpe
865
New contributor
James Thorpe is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
1
Do the test in water
– user6760
Dec 20 at 1:21
Then what doeskilogrammean?
– Mawg
Dec 20 at 7:13
3
You may want to distinguish weight (the gravitational force on an object) from apparent weight (the gravitational force minus the buoyant force). Weight is directly proportional to mass, but apparent weight depends on density and the properties of the surrounding fluid.
– J. Murray
Dec 20 at 7:22
1
As @J.Murray mentions, the entire premise (or the wording) here is actually wrong. For 1 kg steel and 1 kg feathers, both the masses $m$ will be equal and the weights $w=mg$ will be equal, since $$m_1=m_2quad text{and}quad w_1=m_1g=m_2g=w_2$$ But as is clear from the context, you don't really mean physical weight, you rather mean total force (or apparent weight, if you will, as J.Murray suggests). With a tipping scale you namely get a comparison of the total force on the objects and not a comparison of their weights.
– Steeven
Dec 20 at 8:35
@Steeven. To be fair, the meaning of the word weight is just not standardized. There are plenty of people who use weight to mean force due to gravity, and there are plenty of people who use weight to mean what is read by a force scale.
– march
Dec 20 at 21:11
add a comment |
I'm having a discussion at the moment regarding the mass of $1,{rm kg}$ of feathers and $1,{rm kg}$ of steel.
The person I'm arguing with states that $1,{rm kg}$ of feathers will be lighter when weighed, compared to the $1,{rm kg}$ of steel, because the feathers are more buoyant.
She has done her calculations for the density of feathers and works out that $1,{rm kg}$ of feathers will displace $400,{rm L}$ of water. And because the feathers are more buoyant than the steel they would actually weigh less.
We are talking about the kilogram as a unit of mass, not weight.
The argument goes that if a scale is balanced perfectly in vacuum, it is not in air. The feathers, being more buoyant in air, would cause the scale to tip toward the steel.
I’m sure she is wrong, and even though buoyancy may be a factor, she calculates that the feathers would only have about half the weight in air that they do in vacuum.
So the question is, if equal masses of feathers and steel were balanced in a vacuum, would they still balance in $1,{rm atm}$? If not, what would be the difference in weight?
mass density fluid-statics buoyancy weight
edited Dec 20 at 8:11
Qmechanic♦
101k121831142
asked Dec 20 at 0:29
James Thorpe
865
New contributor
James Thorpe is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
I'm having a discussion at the moment regarding the mass of $1,{rm kg}$ of feathers and $1,{rm kg}$ of steel.
The person I'm arguing with states that $1,{rm kg}$ of feathers will be lighter when weighed, compared to the $1,{rm kg}$ of steel, because the feathers are more buoyant.
She has done her calculations for the density of feathers and works out that $1,{rm kg}$ of feathers will displace $400,{rm L}$ of water. And because the feathers are more buoyant than the steel they would actually weigh less.
We are talking about the kilogram as a unit of mass, not weight.
The argument goes that if a scale is balanced perfectly in vacuum, it is not in air. The feathers, being more buoyant in air, would cause the scale to tip toward the steel.
I’m sure she is wrong, and even though buoyancy may be a factor, she calculates that the feathers would only have about half the weight in air that they do in vacuum.
So the question is, if equal masses of feathers and steel were balanced in a vacuum, would they still balance in $1,{rm atm}$? If not, what would be the difference in weight?
mass density fluid-statics buoyancy weight
mass density fluid-statics buoyancy weight
edited Dec 20 at 8:11
Qmechanic♦
101k121831142
asked Dec 20 at 0:29
James Thorpe
865
New contributor
James Thorpe is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
edited Dec 20 at 8:11
Qmechanic♦
101k121831142
asked Dec 20 at 0:29
James Thorpe
865
New contributor
James Thorpe is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
edited Dec 20 at 8:11
Qmechanic♦
101k121831142
edited Dec 20 at 8:11
Qmechanic♦
101k121831142
edited Dec 20 at 8:11
Qmechanic♦
101k121831142
101k121831142
asked Dec 20 at 0:29
James Thorpe
865
New contributor
James Thorpe is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
asked Dec 20 at 0:29
James Thorpe
865
asked Dec 20 at 0:29
James Thorpe
865
865
New contributor
James Thorpe is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
New contributor
James Thorpe is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
James Thorpe is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
1
Do the test in water
– user6760
Dec 20 at 1:21
Then what doeskilogrammean?
– Mawg
Dec 20 at 7:13
3
You may want to distinguish weight (the gravitational force on an object) from apparent weight (the gravitational force minus the buoyant force). Weight is directly proportional to mass, but apparent weight depends on density and the properties of the surrounding fluid.
– J. Murray
Dec 20 at 7:22
1
As @J.Murray mentions, the entire premise (or the wording) here is actually wrong. For 1 kg steel and 1 kg feathers, both the masses $m$ will be equal and the weights $w=mg$ will be equal, since $$m_1=m_2quad text{and}quad w_1=m_1g=m_2g=w_2$$ But as is clear from the context, you don't really mean physical weight, you rather mean total force (or apparent weight, if you will, as J.Murray suggests). With a tipping scale you namely get a comparison of the total force on the objects and not a comparison of their weights.
– Steeven
Dec 20 at 8:35
@Steeven. To be fair, the meaning of the word weight is just not standardized. There are plenty of people who use weight to mean force due to gravity, and there are plenty of people who use weight to mean what is read by a force scale.
– march
Dec 20 at 21:11
add a comment |
1
Do the test in water
– user6760
Dec 20 at 1:21
Then what doeskilogrammean?
– Mawg
Dec 20 at 7:13
3
You may want to distinguish weight (the gravitational force on an object) from apparent weight (the gravitational force minus the buoyant force). Weight is directly proportional to mass, but apparent weight depends on density and the properties of the surrounding fluid.
– J. Murray
Dec 20 at 7:22
1
As @J.Murray mentions, the entire premise (or the wording) here is actually wrong. For 1 kg steel and 1 kg feathers, both the masses $m$ will be equal and the weights $w=mg$ will be equal, since $$m_1=m_2quad text{and}quad w_1=m_1g=m_2g=w_2$$ But as is clear from the context, you don't really mean physical weight, you rather mean total force (or apparent weight, if you will, as J.Murray suggests). With a tipping scale you namely get a comparison of the total force on the objects and not a comparison of their weights.
– Steeven
Dec 20 at 8:35
@Steeven. To be fair, the meaning of the word weight is just not standardized. There are plenty of people who use weight to mean force due to gravity, and there are plenty of people who use weight to mean what is read by a force scale.
– march
Dec 20 at 21:11
1
1
Do the test in water
– user6760
Dec 20 at 1:21
Do the test in water
– user6760
Dec 20 at 1:21
Then what does
kilogram mean?– Mawg
Dec 20 at 7:13
Then what does
kilogram mean?– Mawg
Dec 20 at 7:13
3
3
You may want to distinguish weight (the gravitational force on an object) from apparent weight (the gravitational force minus the buoyant force). Weight is directly proportional to mass, but apparent weight depends on density and the properties of the surrounding fluid.
– J. Murray
Dec 20 at 7:22
You may want to distinguish weight (the gravitational force on an object) from apparent weight (the gravitational force minus the buoyant force). Weight is directly proportional to mass, but apparent weight depends on density and the properties of the surrounding fluid.
– J. Murray
Dec 20 at 7:22
1
1
As @J.Murray mentions, the entire premise (or the wording) here is actually wrong. For 1 kg steel and 1 kg feathers, both the masses $m$ will be equal and the weights $w=mg$ will be equal, since $$m_1=m_2quad text{and}quad w_1=m_1g=m_2g=w_2$$ But as is clear from the context, you don't really mean physical weight, you rather mean total force (or apparent weight, if you will, as J.Murray suggests). With a tipping scale you namely get a comparison of the total force on the objects and not a comparison of their weights.
– Steeven
Dec 20 at 8:35
As @J.Murray mentions, the entire premise (or the wording) here is actually wrong. For 1 kg steel and 1 kg feathers, both the masses $m$ will be equal and the weights $w=mg$ will be equal, since $$m_1=m_2quad text{and}quad w_1=m_1g=m_2g=w_2$$ But as is clear from the context, you don't really mean physical weight, you rather mean total force (or apparent weight, if you will, as J.Murray suggests). With a tipping scale you namely get a comparison of the total force on the objects and not a comparison of their weights.
– Steeven
Dec 20 at 8:35
@Steeven. To be fair, the meaning of the word weight is just not standardized. There are plenty of people who use weight to mean force due to gravity, and there are plenty of people who use weight to mean what is read by a force scale.
– march
Dec 20 at 21:11
@Steeven. To be fair, the meaning of the word weight is just not standardized. There are plenty of people who use weight to mean force due to gravity, and there are plenty of people who use weight to mean what is read by a force scale.
– march
Dec 20 at 21:11
add a comment |
3 Answers
3
active
oldest
votes
Feathers are made from keratin, with a density of about 1.3 grams/cc. The net volume displaced by a kilogram of feathers is then 751 cc's. Steel has a density of 7.86 g/cc and a kilogram of it displaces 127 cc's.
Sea level air has a density of 0.0012 g/cc, so the buoyant force on 751 cc's of keratin is then 0.92 grams and the buoyant force on 127 cc's of steel is 0.155 grams.
This means that if we weigh the keratin in a vacuum, it will weigh 1 kilogram but in air it will weigh 999.08 grams. If we weigh the steel in a vacuum, it will weigh 1 kilogram but in air it will weigh 999.85 grams.
If we place the two bodies- keratin and steel, one kg of each- on a pivoting balance equidistant from the pivot in a vacuum, they will be in balance, but in air the keratin will be lighter by (999.85-999.08) grams or 0.77 grams.
answered Dec 20 at 4:40
niels nielsen
15.8k42651
3
A heap of feathers has smaller density than keratin: there are lots of voids at least between individual feathers.
– Ruslan
Dec 20 at 6:48
Thank you sir, this is exactly the reply I expected and had hoped to get.
– James Thorpe
Dec 20 at 7:11
6
@Ruslan In the atmosphere, those voids would be filled with neutrally-buoyant air.
– Glurth
Dec 20 at 7:14
Though I understand it, I don’t like using kg as a unit for weight like you did in paragraph 3.
– Dave
Dec 20 at 16:49
1
@dave, I was wondering when someone would call me out for that. I did it because I'm a recovering ex-engineer who has just embarked on the 12-step program. Step 4 is to learn the distinction between mass and weight and I'm not there yet.
– niels nielsen
Dec 20 at 16:51
add a comment |
You can argue that her feathers contain a lot of air (never perfectly compressed) so that the volume is not true. You could also take your kilo of steel and make it into a hollow sphere (empty) of the same volume as the feathers (whatever that is) and then it would actually weigh less because you have no air inside. But you could be nice and let her make the point about a bigger volume being more bouyant, she is correct about this but ..... even volume is tricky.
answered Dec 20 at 1:19
PhysicsDave
82636
The argument is not that buoyancy makes no difference, it is the amount of difference it makes that is in dispute. She is arguing that the buoyancy of the feathers is related to the density of feathers in the bag including the air within. This I do not agree with. She calculates that it’s buoyancy would make it weigh almost half of what it would weigh in a vacuum. I do not agree with either of these figures.
– James Thorpe
Dec 20 at 3:03
1
She also calculates that a mass of one kilogram of feathers would displace 400 litres of water if immersed. I do not agree with that figure either.
– James Thorpe
Dec 20 at 3:12
Yes good points.
– PhysicsDave
Dec 20 at 5:25
add a comment |
Due to buoyancy one kilogram of hot air balloon indeed weighs less than one kilogram of steel!
answered Dec 20 at 0:43
my2cts
4,4482617
It would be nice if you told us by how much, haha.
– FGSUZ
Dec 20 at 0:45
3
This would be better as a comment
– Aaron Stevens
Dec 20 at 1:22
I have used the above to elaborate on the question. Thanks.
– James Thorpe
Dec 20 at 1:28
@AaronStevens I disagree that this should have been posted as a comment --- it's not a request for clarification or a suggestion for improvement to the question. (I'm glad it resulted in an improvement to the question anyway.) It's more like an answer, since it sets up a very clear example of a case where the buoyant force results in a different "weight" for two objects with the same mass, but it's so terse that it's not a terribly useful answer. It's fun, but here on Stack Exchange our relationship to fun is nuanced.
– rob♦
Dec 21 at 4:03
add a comment |
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3 Answers
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active
oldest
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3 Answers
3
active
oldest
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Feathers are made from keratin, with a density of about 1.3 grams/cc. The net volume displaced by a kilogram of feathers is then 751 cc's. Steel has a density of 7.86 g/cc and a kilogram of it displaces 127 cc's.
Sea level air has a density of 0.0012 g/cc, so the buoyant force on 751 cc's of keratin is then 0.92 grams and the buoyant force on 127 cc's of steel is 0.155 grams.
This means that if we weigh the keratin in a vacuum, it will weigh 1 kilogram but in air it will weigh 999.08 grams. If we weigh the steel in a vacuum, it will weigh 1 kilogram but in air it will weigh 999.85 grams.
If we place the two bodies- keratin and steel, one kg of each- on a pivoting balance equidistant from the pivot in a vacuum, they will be in balance, but in air the keratin will be lighter by (999.85-999.08) grams or 0.77 grams.
answered Dec 20 at 4:40
niels nielsen
15.8k42651
3
A heap of feathers has smaller density than keratin: there are lots of voids at least between individual feathers.
– Ruslan
Dec 20 at 6:48
Thank you sir, this is exactly the reply I expected and had hoped to get.
– James Thorpe
Dec 20 at 7:11
6
@Ruslan In the atmosphere, those voids would be filled with neutrally-buoyant air.
– Glurth
Dec 20 at 7:14
Though I understand it, I don’t like using kg as a unit for weight like you did in paragraph 3.
– Dave
Dec 20 at 16:49
1
@dave, I was wondering when someone would call me out for that. I did it because I'm a recovering ex-engineer who has just embarked on the 12-step program. Step 4 is to learn the distinction between mass and weight and I'm not there yet.
– niels nielsen
Dec 20 at 16:51
add a comment |
Feathers are made from keratin, with a density of about 1.3 grams/cc. The net volume displaced by a kilogram of feathers is then 751 cc's. Steel has a density of 7.86 g/cc and a kilogram of it displaces 127 cc's.
Sea level air has a density of 0.0012 g/cc, so the buoyant force on 751 cc's of keratin is then 0.92 grams and the buoyant force on 127 cc's of steel is 0.155 grams.
This means that if we weigh the keratin in a vacuum, it will weigh 1 kilogram but in air it will weigh 999.08 grams. If we weigh the steel in a vacuum, it will weigh 1 kilogram but in air it will weigh 999.85 grams.
If we place the two bodies- keratin and steel, one kg of each- on a pivoting balance equidistant from the pivot in a vacuum, they will be in balance, but in air the keratin will be lighter by (999.85-999.08) grams or 0.77 grams.
answered Dec 20 at 4:40
niels nielsen
15.8k42651
3
A heap of feathers has smaller density than keratin: there are lots of voids at least between individual feathers.
– Ruslan
Dec 20 at 6:48
Thank you sir, this is exactly the reply I expected and had hoped to get.
– James Thorpe
Dec 20 at 7:11
6
@Ruslan In the atmosphere, those voids would be filled with neutrally-buoyant air.
– Glurth
Dec 20 at 7:14
Though I understand it, I don’t like using kg as a unit for weight like you did in paragraph 3.
– Dave
Dec 20 at 16:49
1
@dave, I was wondering when someone would call me out for that. I did it because I'm a recovering ex-engineer who has just embarked on the 12-step program. Step 4 is to learn the distinction between mass and weight and I'm not there yet.
– niels nielsen
Dec 20 at 16:51
add a comment |
Feathers are made from keratin, with a density of about 1.3 grams/cc. The net volume displaced by a kilogram of feathers is then 751 cc's. Steel has a density of 7.86 g/cc and a kilogram of it displaces 127 cc's.
Sea level air has a density of 0.0012 g/cc, so the buoyant force on 751 cc's of keratin is then 0.92 grams and the buoyant force on 127 cc's of steel is 0.155 grams.
This means that if we weigh the keratin in a vacuum, it will weigh 1 kilogram but in air it will weigh 999.08 grams. If we weigh the steel in a vacuum, it will weigh 1 kilogram but in air it will weigh 999.85 grams.
If we place the two bodies- keratin and steel, one kg of each- on a pivoting balance equidistant from the pivot in a vacuum, they will be in balance, but in air the keratin will be lighter by (999.85-999.08) grams or 0.77 grams.
answered Dec 20 at 4:40
niels nielsen
15.8k42651
Feathers are made from keratin, with a density of about 1.3 grams/cc. The net volume displaced by a kilogram of feathers is then 751 cc's. Steel has a density of 7.86 g/cc and a kilogram of it displaces 127 cc's.
Sea level air has a density of 0.0012 g/cc, so the buoyant force on 751 cc's of keratin is then 0.92 grams and the buoyant force on 127 cc's of steel is 0.155 grams.
This means that if we weigh the keratin in a vacuum, it will weigh 1 kilogram but in air it will weigh 999.08 grams. If we weigh the steel in a vacuum, it will weigh 1 kilogram but in air it will weigh 999.85 grams.
If we place the two bodies- keratin and steel, one kg of each- on a pivoting balance equidistant from the pivot in a vacuum, they will be in balance, but in air the keratin will be lighter by (999.85-999.08) grams or 0.77 grams.
answered Dec 20 at 4:40
niels nielsen
15.8k42651
edited Dec 20 at 4:55
answered Dec 20 at 4:40
niels nielsen
15.8k42651
answered Dec 20 at 4:40
niels nielsen
15.8k42651
answered Dec 20 at 4:40
niels nielsen
15.8k42651
15.8k42651
3
A heap of feathers has smaller density than keratin: there are lots of voids at least between individual feathers.
– Ruslan
Dec 20 at 6:48
Thank you sir, this is exactly the reply I expected and had hoped to get.
– James Thorpe
Dec 20 at 7:11
6
@Ruslan In the atmosphere, those voids would be filled with neutrally-buoyant air.
– Glurth
Dec 20 at 7:14
Though I understand it, I don’t like using kg as a unit for weight like you did in paragraph 3.
– Dave
Dec 20 at 16:49
1
@dave, I was wondering when someone would call me out for that. I did it because I'm a recovering ex-engineer who has just embarked on the 12-step program. Step 4 is to learn the distinction between mass and weight and I'm not there yet.
– niels nielsen
Dec 20 at 16:51
add a comment |
3
A heap of feathers has smaller density than keratin: there are lots of voids at least between individual feathers.
– Ruslan
Dec 20 at 6:48
Thank you sir, this is exactly the reply I expected and had hoped to get.
– James Thorpe
Dec 20 at 7:11
6
@Ruslan In the atmosphere, those voids would be filled with neutrally-buoyant air.
– Glurth
Dec 20 at 7:14
Though I understand it, I don’t like using kg as a unit for weight like you did in paragraph 3.
– Dave
Dec 20 at 16:49
1
@dave, I was wondering when someone would call me out for that. I did it because I'm a recovering ex-engineer who has just embarked on the 12-step program. Step 4 is to learn the distinction between mass and weight and I'm not there yet.
– niels nielsen
Dec 20 at 16:51
3
3
A heap of feathers has smaller density than keratin: there are lots of voids at least between individual feathers.
– Ruslan
Dec 20 at 6:48
A heap of feathers has smaller density than keratin: there are lots of voids at least between individual feathers.
– Ruslan
Dec 20 at 6:48
Thank you sir, this is exactly the reply I expected and had hoped to get.
– James Thorpe
Dec 20 at 7:11
Thank you sir, this is exactly the reply I expected and had hoped to get.
– James Thorpe
Dec 20 at 7:11
6
6
@Ruslan In the atmosphere, those voids would be filled with neutrally-buoyant air.
– Glurth
Dec 20 at 7:14
@Ruslan In the atmosphere, those voids would be filled with neutrally-buoyant air.
– Glurth
Dec 20 at 7:14
Though I understand it, I don’t like using kg as a unit for weight like you did in paragraph 3.
– Dave
Dec 20 at 16:49
Though I understand it, I don’t like using kg as a unit for weight like you did in paragraph 3.
– Dave
Dec 20 at 16:49
1
1
@dave, I was wondering when someone would call me out for that. I did it because I'm a recovering ex-engineer who has just embarked on the 12-step program. Step 4 is to learn the distinction between mass and weight and I'm not there yet.
– niels nielsen
Dec 20 at 16:51
@dave, I was wondering when someone would call me out for that. I did it because I'm a recovering ex-engineer who has just embarked on the 12-step program. Step 4 is to learn the distinction between mass and weight and I'm not there yet.
– niels nielsen
Dec 20 at 16:51
add a comment |
You can argue that her feathers contain a lot of air (never perfectly compressed) so that the volume is not true. You could also take your kilo of steel and make it into a hollow sphere (empty) of the same volume as the feathers (whatever that is) and then it would actually weigh less because you have no air inside. But you could be nice and let her make the point about a bigger volume being more bouyant, she is correct about this but ..... even volume is tricky.
answered Dec 20 at 1:19
PhysicsDave
82636
The argument is not that buoyancy makes no difference, it is the amount of difference it makes that is in dispute. She is arguing that the buoyancy of the feathers is related to the density of feathers in the bag including the air within. This I do not agree with. She calculates that it’s buoyancy would make it weigh almost half of what it would weigh in a vacuum. I do not agree with either of these figures.
– James Thorpe
Dec 20 at 3:03
1
She also calculates that a mass of one kilogram of feathers would displace 400 litres of water if immersed. I do not agree with that figure either.
– James Thorpe
Dec 20 at 3:12
Yes good points.
– PhysicsDave
Dec 20 at 5:25
add a comment |
You can argue that her feathers contain a lot of air (never perfectly compressed) so that the volume is not true. You could also take your kilo of steel and make it into a hollow sphere (empty) of the same volume as the feathers (whatever that is) and then it would actually weigh less because you have no air inside. But you could be nice and let her make the point about a bigger volume being more bouyant, she is correct about this but ..... even volume is tricky.
answered Dec 20 at 1:19
PhysicsDave
82636
The argument is not that buoyancy makes no difference, it is the amount of difference it makes that is in dispute. She is arguing that the buoyancy of the feathers is related to the density of feathers in the bag including the air within. This I do not agree with. She calculates that it’s buoyancy would make it weigh almost half of what it would weigh in a vacuum. I do not agree with either of these figures.
– James Thorpe
Dec 20 at 3:03
1
She also calculates that a mass of one kilogram of feathers would displace 400 litres of water if immersed. I do not agree with that figure either.
– James Thorpe
Dec 20 at 3:12
Yes good points.
– PhysicsDave
Dec 20 at 5:25
add a comment |
You can argue that her feathers contain a lot of air (never perfectly compressed) so that the volume is not true. You could also take your kilo of steel and make it into a hollow sphere (empty) of the same volume as the feathers (whatever that is) and then it would actually weigh less because you have no air inside. But you could be nice and let her make the point about a bigger volume being more bouyant, she is correct about this but ..... even volume is tricky.
answered Dec 20 at 1:19
PhysicsDave
82636
You can argue that her feathers contain a lot of air (never perfectly compressed) so that the volume is not true. You could also take your kilo of steel and make it into a hollow sphere (empty) of the same volume as the feathers (whatever that is) and then it would actually weigh less because you have no air inside. But you could be nice and let her make the point about a bigger volume being more bouyant, she is correct about this but ..... even volume is tricky.
answered Dec 20 at 1:19
PhysicsDave
82636
answered Dec 20 at 1:19
PhysicsDave
82636
answered Dec 20 at 1:19
PhysicsDave
82636
answered Dec 20 at 1:19
PhysicsDave
82636
82636
The argument is not that buoyancy makes no difference, it is the amount of difference it makes that is in dispute. She is arguing that the buoyancy of the feathers is related to the density of feathers in the bag including the air within. This I do not agree with. She calculates that it’s buoyancy would make it weigh almost half of what it would weigh in a vacuum. I do not agree with either of these figures.
– James Thorpe
Dec 20 at 3:03
1
She also calculates that a mass of one kilogram of feathers would displace 400 litres of water if immersed. I do not agree with that figure either.
– James Thorpe
Dec 20 at 3:12
Yes good points.
– PhysicsDave
Dec 20 at 5:25
add a comment |
The argument is not that buoyancy makes no difference, it is the amount of difference it makes that is in dispute. She is arguing that the buoyancy of the feathers is related to the density of feathers in the bag including the air within. This I do not agree with. She calculates that it’s buoyancy would make it weigh almost half of what it would weigh in a vacuum. I do not agree with either of these figures.
– James Thorpe
Dec 20 at 3:03
1
She also calculates that a mass of one kilogram of feathers would displace 400 litres of water if immersed. I do not agree with that figure either.
– James Thorpe
Dec 20 at 3:12
Yes good points.
– PhysicsDave
Dec 20 at 5:25
The argument is not that buoyancy makes no difference, it is the amount of difference it makes that is in dispute. She is arguing that the buoyancy of the feathers is related to the density of feathers in the bag including the air within. This I do not agree with. She calculates that it’s buoyancy would make it weigh almost half of what it would weigh in a vacuum. I do not agree with either of these figures.
– James Thorpe
Dec 20 at 3:03
The argument is not that buoyancy makes no difference, it is the amount of difference it makes that is in dispute. She is arguing that the buoyancy of the feathers is related to the density of feathers in the bag including the air within. This I do not agree with. She calculates that it’s buoyancy would make it weigh almost half of what it would weigh in a vacuum. I do not agree with either of these figures.
– James Thorpe
Dec 20 at 3:03
1
1
She also calculates that a mass of one kilogram of feathers would displace 400 litres of water if immersed. I do not agree with that figure either.
– James Thorpe
Dec 20 at 3:12
She also calculates that a mass of one kilogram of feathers would displace 400 litres of water if immersed. I do not agree with that figure either.
– James Thorpe
Dec 20 at 3:12
Yes good points.
– PhysicsDave
Dec 20 at 5:25
Yes good points.
– PhysicsDave
Dec 20 at 5:25
add a comment |
Due to buoyancy one kilogram of hot air balloon indeed weighs less than one kilogram of steel!
answered Dec 20 at 0:43
my2cts
4,4482617
It would be nice if you told us by how much, haha.
– FGSUZ
Dec 20 at 0:45
3
This would be better as a comment
– Aaron Stevens
Dec 20 at 1:22
I have used the above to elaborate on the question. Thanks.
– James Thorpe
Dec 20 at 1:28
@AaronStevens I disagree that this should have been posted as a comment --- it's not a request for clarification or a suggestion for improvement to the question. (I'm glad it resulted in an improvement to the question anyway.) It's more like an answer, since it sets up a very clear example of a case where the buoyant force results in a different "weight" for two objects with the same mass, but it's so terse that it's not a terribly useful answer. It's fun, but here on Stack Exchange our relationship to fun is nuanced.
– rob♦
Dec 21 at 4:03
add a comment |
Due to buoyancy one kilogram of hot air balloon indeed weighs less than one kilogram of steel!
answered Dec 20 at 0:43
my2cts
4,4482617
It would be nice if you told us by how much, haha.
– FGSUZ
Dec 20 at 0:45
3
This would be better as a comment
– Aaron Stevens
Dec 20 at 1:22
I have used the above to elaborate on the question. Thanks.
– James Thorpe
Dec 20 at 1:28
@AaronStevens I disagree that this should have been posted as a comment --- it's not a request for clarification or a suggestion for improvement to the question. (I'm glad it resulted in an improvement to the question anyway.) It's more like an answer, since it sets up a very clear example of a case where the buoyant force results in a different "weight" for two objects with the same mass, but it's so terse that it's not a terribly useful answer. It's fun, but here on Stack Exchange our relationship to fun is nuanced.
– rob♦
Dec 21 at 4:03
add a comment |
Due to buoyancy one kilogram of hot air balloon indeed weighs less than one kilogram of steel!
answered Dec 20 at 0:43
my2cts
4,4482617
Due to buoyancy one kilogram of hot air balloon indeed weighs less than one kilogram of steel!
answered Dec 20 at 0:43
my2cts
4,4482617
answered Dec 20 at 0:43
my2cts
4,4482617
answered Dec 20 at 0:43
my2cts
4,4482617
answered Dec 20 at 0:43
my2cts
4,4482617
4,4482617
It would be nice if you told us by how much, haha.
– FGSUZ
Dec 20 at 0:45
3
This would be better as a comment
– Aaron Stevens
Dec 20 at 1:22
I have used the above to elaborate on the question. Thanks.
– James Thorpe
Dec 20 at 1:28
@AaronStevens I disagree that this should have been posted as a comment --- it's not a request for clarification or a suggestion for improvement to the question. (I'm glad it resulted in an improvement to the question anyway.) It's more like an answer, since it sets up a very clear example of a case where the buoyant force results in a different "weight" for two objects with the same mass, but it's so terse that it's not a terribly useful answer. It's fun, but here on Stack Exchange our relationship to fun is nuanced.
– rob♦
Dec 21 at 4:03
add a comment |
It would be nice if you told us by how much, haha.
– FGSUZ
Dec 20 at 0:45
3
This would be better as a comment
– Aaron Stevens
Dec 20 at 1:22
I have used the above to elaborate on the question. Thanks.
– James Thorpe
Dec 20 at 1:28
@AaronStevens I disagree that this should have been posted as a comment --- it's not a request for clarification or a suggestion for improvement to the question. (I'm glad it resulted in an improvement to the question anyway.) It's more like an answer, since it sets up a very clear example of a case where the buoyant force results in a different "weight" for two objects with the same mass, but it's so terse that it's not a terribly useful answer. It's fun, but here on Stack Exchange our relationship to fun is nuanced.
– rob♦
Dec 21 at 4:03
It would be nice if you told us by how much, haha.
– FGSUZ
Dec 20 at 0:45
It would be nice if you told us by how much, haha.
– FGSUZ
Dec 20 at 0:45
3
3
This would be better as a comment
– Aaron Stevens
Dec 20 at 1:22
This would be better as a comment
– Aaron Stevens
Dec 20 at 1:22
I have used the above to elaborate on the question. Thanks.
– James Thorpe
Dec 20 at 1:28
I have used the above to elaborate on the question. Thanks.
– James Thorpe
Dec 20 at 1:28
@AaronStevens I disagree that this should have been posted as a comment --- it's not a request for clarification or a suggestion for improvement to the question. (I'm glad it resulted in an improvement to the question anyway.) It's more like an answer, since it sets up a very clear example of a case where the buoyant force results in a different "weight" for two objects with the same mass, but it's so terse that it's not a terribly useful answer. It's fun, but here on Stack Exchange our relationship to fun is nuanced.
– rob♦
Dec 21 at 4:03
@AaronStevens I disagree that this should have been posted as a comment --- it's not a request for clarification or a suggestion for improvement to the question. (I'm glad it resulted in an improvement to the question anyway.) It's more like an answer, since it sets up a very clear example of a case where the buoyant force results in a different "weight" for two objects with the same mass, but it's so terse that it's not a terribly useful answer. It's fun, but here on Stack Exchange our relationship to fun is nuanced.
– rob♦
Dec 21 at 4:03
add a comment |
James Thorpe is a new contributor. Be nice, and check out our Code of Conduct.
James Thorpe is a new contributor. Be nice, and check out our Code of Conduct.
James Thorpe is a new contributor. Be nice, and check out our Code of Conduct.
James Thorpe is a new contributor. Be nice, and check out our Code of Conduct.
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1
Do the test in water
– user6760
Dec 20 at 1:21
Then what does
kilogrammean?– Mawg
Dec 20 at 7:13
3
You may want to distinguish weight (the gravitational force on an object) from apparent weight (the gravitational force minus the buoyant force). Weight is directly proportional to mass, but apparent weight depends on density and the properties of the surrounding fluid.
– J. Murray
Dec 20 at 7:22
1
As @J.Murray mentions, the entire premise (or the wording) here is actually wrong. For 1 kg steel and 1 kg feathers, both the masses $m$ will be equal and the weights $w=mg$ will be equal, since $$m_1=m_2quad text{and}quad w_1=m_1g=m_2g=w_2$$ But as is clear from the context, you don't really mean physical weight, you rather mean total force (or apparent weight, if you will, as J.Murray suggests). With a tipping scale you namely get a comparison of the total force on the objects and not a comparison of their weights.
– Steeven
Dec 20 at 8:35
@Steeven. To be fair, the meaning of the word weight is just not standardized. There are plenty of people who use weight to mean force due to gravity, and there are plenty of people who use weight to mean what is read by a force scale.
– march
Dec 20 at 21:11