Why doesn't a table tennis ball float on a surface of steel balls? How do we calculate buoyancy here?












14












$begingroup$


Place the beaker full of steel balls and submerge the table tennis ball under the steel balls. The table tennis ball does not float up. Why does it not float up? Do table tennis balls float when the diameter of steel balls is reduced? How to calculate the buoyancy of steel balls?



A submerged table tennis ball










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$endgroup$








  • 10




    $begingroup$
    Shake the bowl a little. Like Brownian motion on water molecules.
    $endgroup$
    – Hot Licks
    2 days ago
















14












$begingroup$


Place the beaker full of steel balls and submerge the table tennis ball under the steel balls. The table tennis ball does not float up. Why does it not float up? Do table tennis balls float when the diameter of steel balls is reduced? How to calculate the buoyancy of steel balls?



A submerged table tennis ball










share|cite|improve this question











$endgroup$








  • 10




    $begingroup$
    Shake the bowl a little. Like Brownian motion on water molecules.
    $endgroup$
    – Hot Licks
    2 days ago














14












14








14


4



$begingroup$


Place the beaker full of steel balls and submerge the table tennis ball under the steel balls. The table tennis ball does not float up. Why does it not float up? Do table tennis balls float when the diameter of steel balls is reduced? How to calculate the buoyancy of steel balls?



A submerged table tennis ball










share|cite|improve this question











$endgroup$




Place the beaker full of steel balls and submerge the table tennis ball under the steel balls. The table tennis ball does not float up. Why does it not float up? Do table tennis balls float when the diameter of steel balls is reduced? How to calculate the buoyancy of steel balls?



A submerged table tennis ball







newtonian-mechanics forces classical-mechanics fluid-dynamics






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share|cite|improve this question













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share|cite|improve this question








edited yesterday









curiousdannii

612614




612614










asked 2 days ago









enbin zhengenbin zheng

9417




9417








  • 10




    $begingroup$
    Shake the bowl a little. Like Brownian motion on water molecules.
    $endgroup$
    – Hot Licks
    2 days ago














  • 10




    $begingroup$
    Shake the bowl a little. Like Brownian motion on water molecules.
    $endgroup$
    – Hot Licks
    2 days ago








10




10




$begingroup$
Shake the bowl a little. Like Brownian motion on water molecules.
$endgroup$
– Hot Licks
2 days ago




$begingroup$
Shake the bowl a little. Like Brownian motion on water molecules.
$endgroup$
– Hot Licks
2 days ago










2 Answers
2






active

oldest

votes


















39












$begingroup$

The ball bearings are behaving as a solid because the forces between the steel balls (i.e. friction) are large enough to hold the balls in position relative to each other.



If you apply enough force to a solid you will cause it to fracture or to cause plastic flow. So for example if you attached a string to the ball and pulled upwards with enough force it would cause the steel balls to flow over each other and the table tennis ball would move up. The force required is related to the yield stress of the solid formed by the steel balls.



You can make the steel balls behave as a fluid by making a gas flow through them. This creates a fluidised bed. The gas pushes the steel balls apart so the friction between them is removed, and in this state the steel balls will behave like a fluid and the table tennis ball would float upwards.



Alternatively just shake the beaker. This is equivalent to adding thermal energy i.e. heating the system until it melts. If you shake the beaker you'll find the table tennis ball floats upwards.






share|cite|improve this answer











$endgroup$









  • 7




    $begingroup$
    -1 Shaking the beaker will make the ball float upwards because of the size difference, not the density difference. It's a very different effect. (If you were to reverse the materials of the balls - it is the large steel ball that would end up on top)
    $endgroup$
    – UKMonkey
    2 days ago








  • 4




    $begingroup$
    *edit density and size (in effect density including air gaps) here's the wiki article en.wikipedia.org/wiki/Granular_convection
    $endgroup$
    – UKMonkey
    2 days ago








  • 12




    $begingroup$
    @UKMonkey it's both size and density. As it happens I explained the effect of size in Why do the big nuts always remain at top? The Brazil-nut Effect.
    $endgroup$
    – John Rennie
    2 days ago






  • 3




    $begingroup$
    The linked answer is good but it doesn't explain how buoyancy is different. I've actually never seen a good explanation of the mechanisms underlying buoyancy. Often it's described as if it were a fundamental force.
    $endgroup$
    – JimmyJames
    2 days ago






  • 2




    $begingroup$
    Another comparison that would be interesting is how this is or is not related to how a dust particle can hover in still air for a period that is longer than would be expected given relative density to air.
    $endgroup$
    – JimmyJames
    2 days ago



















2












$begingroup$

Well, what if the steel balls were extremely small, say molecular size. In that case, the constraining annulus would look like a polished steel collar, and would likely hold down the ball even if the glass was shattered, underwater, in a swimming pool. . (The van der Waals forces, and metallic bonds, would account for that.)



But this example given, shows discreet balls of intermediate size, and unless they are magnetized, their coupling with the container is what allows restraint of the tennis ball.
If THIS setup was in the bottom of a deeper pool, and the beaker was shattered,
the steel balls would run radially away, and the tennis ball would pop up.



(Note: I answered this as if there was water in the beaker along with the steel balls and tennis ball. But the answer is not changed by my error.)



The term "van der Waals force" is sometimes used loosely for all intermolecular forces.






share|cite|improve this answer










New contributor




David Newell is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.






$endgroup$













  • $begingroup$
    Can't the steel balls float a table tennis ball even if they are very small?
    $endgroup$
    – enbin zheng
    2 days ago












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2 Answers
2






active

oldest

votes








2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes









39












$begingroup$

The ball bearings are behaving as a solid because the forces between the steel balls (i.e. friction) are large enough to hold the balls in position relative to each other.



If you apply enough force to a solid you will cause it to fracture or to cause plastic flow. So for example if you attached a string to the ball and pulled upwards with enough force it would cause the steel balls to flow over each other and the table tennis ball would move up. The force required is related to the yield stress of the solid formed by the steel balls.



You can make the steel balls behave as a fluid by making a gas flow through them. This creates a fluidised bed. The gas pushes the steel balls apart so the friction between them is removed, and in this state the steel balls will behave like a fluid and the table tennis ball would float upwards.



Alternatively just shake the beaker. This is equivalent to adding thermal energy i.e. heating the system until it melts. If you shake the beaker you'll find the table tennis ball floats upwards.






share|cite|improve this answer











$endgroup$









  • 7




    $begingroup$
    -1 Shaking the beaker will make the ball float upwards because of the size difference, not the density difference. It's a very different effect. (If you were to reverse the materials of the balls - it is the large steel ball that would end up on top)
    $endgroup$
    – UKMonkey
    2 days ago








  • 4




    $begingroup$
    *edit density and size (in effect density including air gaps) here's the wiki article en.wikipedia.org/wiki/Granular_convection
    $endgroup$
    – UKMonkey
    2 days ago








  • 12




    $begingroup$
    @UKMonkey it's both size and density. As it happens I explained the effect of size in Why do the big nuts always remain at top? The Brazil-nut Effect.
    $endgroup$
    – John Rennie
    2 days ago






  • 3




    $begingroup$
    The linked answer is good but it doesn't explain how buoyancy is different. I've actually never seen a good explanation of the mechanisms underlying buoyancy. Often it's described as if it were a fundamental force.
    $endgroup$
    – JimmyJames
    2 days ago






  • 2




    $begingroup$
    Another comparison that would be interesting is how this is or is not related to how a dust particle can hover in still air for a period that is longer than would be expected given relative density to air.
    $endgroup$
    – JimmyJames
    2 days ago
















39












$begingroup$

The ball bearings are behaving as a solid because the forces between the steel balls (i.e. friction) are large enough to hold the balls in position relative to each other.



If you apply enough force to a solid you will cause it to fracture or to cause plastic flow. So for example if you attached a string to the ball and pulled upwards with enough force it would cause the steel balls to flow over each other and the table tennis ball would move up. The force required is related to the yield stress of the solid formed by the steel balls.



You can make the steel balls behave as a fluid by making a gas flow through them. This creates a fluidised bed. The gas pushes the steel balls apart so the friction between them is removed, and in this state the steel balls will behave like a fluid and the table tennis ball would float upwards.



Alternatively just shake the beaker. This is equivalent to adding thermal energy i.e. heating the system until it melts. If you shake the beaker you'll find the table tennis ball floats upwards.






share|cite|improve this answer











$endgroup$









  • 7




    $begingroup$
    -1 Shaking the beaker will make the ball float upwards because of the size difference, not the density difference. It's a very different effect. (If you were to reverse the materials of the balls - it is the large steel ball that would end up on top)
    $endgroup$
    – UKMonkey
    2 days ago








  • 4




    $begingroup$
    *edit density and size (in effect density including air gaps) here's the wiki article en.wikipedia.org/wiki/Granular_convection
    $endgroup$
    – UKMonkey
    2 days ago








  • 12




    $begingroup$
    @UKMonkey it's both size and density. As it happens I explained the effect of size in Why do the big nuts always remain at top? The Brazil-nut Effect.
    $endgroup$
    – John Rennie
    2 days ago






  • 3




    $begingroup$
    The linked answer is good but it doesn't explain how buoyancy is different. I've actually never seen a good explanation of the mechanisms underlying buoyancy. Often it's described as if it were a fundamental force.
    $endgroup$
    – JimmyJames
    2 days ago






  • 2




    $begingroup$
    Another comparison that would be interesting is how this is or is not related to how a dust particle can hover in still air for a period that is longer than would be expected given relative density to air.
    $endgroup$
    – JimmyJames
    2 days ago














39












39








39





$begingroup$

The ball bearings are behaving as a solid because the forces between the steel balls (i.e. friction) are large enough to hold the balls in position relative to each other.



If you apply enough force to a solid you will cause it to fracture or to cause plastic flow. So for example if you attached a string to the ball and pulled upwards with enough force it would cause the steel balls to flow over each other and the table tennis ball would move up. The force required is related to the yield stress of the solid formed by the steel balls.



You can make the steel balls behave as a fluid by making a gas flow through them. This creates a fluidised bed. The gas pushes the steel balls apart so the friction between them is removed, and in this state the steel balls will behave like a fluid and the table tennis ball would float upwards.



Alternatively just shake the beaker. This is equivalent to adding thermal energy i.e. heating the system until it melts. If you shake the beaker you'll find the table tennis ball floats upwards.






share|cite|improve this answer











$endgroup$



The ball bearings are behaving as a solid because the forces between the steel balls (i.e. friction) are large enough to hold the balls in position relative to each other.



If you apply enough force to a solid you will cause it to fracture or to cause plastic flow. So for example if you attached a string to the ball and pulled upwards with enough force it would cause the steel balls to flow over each other and the table tennis ball would move up. The force required is related to the yield stress of the solid formed by the steel balls.



You can make the steel balls behave as a fluid by making a gas flow through them. This creates a fluidised bed. The gas pushes the steel balls apart so the friction between them is removed, and in this state the steel balls will behave like a fluid and the table tennis ball would float upwards.



Alternatively just shake the beaker. This is equivalent to adding thermal energy i.e. heating the system until it melts. If you shake the beaker you'll find the table tennis ball floats upwards.







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited 2 days ago

























answered 2 days ago









John RennieJohn Rennie

279k44557804




279k44557804








  • 7




    $begingroup$
    -1 Shaking the beaker will make the ball float upwards because of the size difference, not the density difference. It's a very different effect. (If you were to reverse the materials of the balls - it is the large steel ball that would end up on top)
    $endgroup$
    – UKMonkey
    2 days ago








  • 4




    $begingroup$
    *edit density and size (in effect density including air gaps) here's the wiki article en.wikipedia.org/wiki/Granular_convection
    $endgroup$
    – UKMonkey
    2 days ago








  • 12




    $begingroup$
    @UKMonkey it's both size and density. As it happens I explained the effect of size in Why do the big nuts always remain at top? The Brazil-nut Effect.
    $endgroup$
    – John Rennie
    2 days ago






  • 3




    $begingroup$
    The linked answer is good but it doesn't explain how buoyancy is different. I've actually never seen a good explanation of the mechanisms underlying buoyancy. Often it's described as if it were a fundamental force.
    $endgroup$
    – JimmyJames
    2 days ago






  • 2




    $begingroup$
    Another comparison that would be interesting is how this is or is not related to how a dust particle can hover in still air for a period that is longer than would be expected given relative density to air.
    $endgroup$
    – JimmyJames
    2 days ago














  • 7




    $begingroup$
    -1 Shaking the beaker will make the ball float upwards because of the size difference, not the density difference. It's a very different effect. (If you were to reverse the materials of the balls - it is the large steel ball that would end up on top)
    $endgroup$
    – UKMonkey
    2 days ago








  • 4




    $begingroup$
    *edit density and size (in effect density including air gaps) here's the wiki article en.wikipedia.org/wiki/Granular_convection
    $endgroup$
    – UKMonkey
    2 days ago








  • 12




    $begingroup$
    @UKMonkey it's both size and density. As it happens I explained the effect of size in Why do the big nuts always remain at top? The Brazil-nut Effect.
    $endgroup$
    – John Rennie
    2 days ago






  • 3




    $begingroup$
    The linked answer is good but it doesn't explain how buoyancy is different. I've actually never seen a good explanation of the mechanisms underlying buoyancy. Often it's described as if it were a fundamental force.
    $endgroup$
    – JimmyJames
    2 days ago






  • 2




    $begingroup$
    Another comparison that would be interesting is how this is or is not related to how a dust particle can hover in still air for a period that is longer than would be expected given relative density to air.
    $endgroup$
    – JimmyJames
    2 days ago








7




7




$begingroup$
-1 Shaking the beaker will make the ball float upwards because of the size difference, not the density difference. It's a very different effect. (If you were to reverse the materials of the balls - it is the large steel ball that would end up on top)
$endgroup$
– UKMonkey
2 days ago






$begingroup$
-1 Shaking the beaker will make the ball float upwards because of the size difference, not the density difference. It's a very different effect. (If you were to reverse the materials of the balls - it is the large steel ball that would end up on top)
$endgroup$
– UKMonkey
2 days ago






4




4




$begingroup$
*edit density and size (in effect density including air gaps) here's the wiki article en.wikipedia.org/wiki/Granular_convection
$endgroup$
– UKMonkey
2 days ago






$begingroup$
*edit density and size (in effect density including air gaps) here's the wiki article en.wikipedia.org/wiki/Granular_convection
$endgroup$
– UKMonkey
2 days ago






12




12




$begingroup$
@UKMonkey it's both size and density. As it happens I explained the effect of size in Why do the big nuts always remain at top? The Brazil-nut Effect.
$endgroup$
– John Rennie
2 days ago




$begingroup$
@UKMonkey it's both size and density. As it happens I explained the effect of size in Why do the big nuts always remain at top? The Brazil-nut Effect.
$endgroup$
– John Rennie
2 days ago




3




3




$begingroup$
The linked answer is good but it doesn't explain how buoyancy is different. I've actually never seen a good explanation of the mechanisms underlying buoyancy. Often it's described as if it were a fundamental force.
$endgroup$
– JimmyJames
2 days ago




$begingroup$
The linked answer is good but it doesn't explain how buoyancy is different. I've actually never seen a good explanation of the mechanisms underlying buoyancy. Often it's described as if it were a fundamental force.
$endgroup$
– JimmyJames
2 days ago




2




2




$begingroup$
Another comparison that would be interesting is how this is or is not related to how a dust particle can hover in still air for a period that is longer than would be expected given relative density to air.
$endgroup$
– JimmyJames
2 days ago




$begingroup$
Another comparison that would be interesting is how this is or is not related to how a dust particle can hover in still air for a period that is longer than would be expected given relative density to air.
$endgroup$
– JimmyJames
2 days ago











2












$begingroup$

Well, what if the steel balls were extremely small, say molecular size. In that case, the constraining annulus would look like a polished steel collar, and would likely hold down the ball even if the glass was shattered, underwater, in a swimming pool. . (The van der Waals forces, and metallic bonds, would account for that.)



But this example given, shows discreet balls of intermediate size, and unless they are magnetized, their coupling with the container is what allows restraint of the tennis ball.
If THIS setup was in the bottom of a deeper pool, and the beaker was shattered,
the steel balls would run radially away, and the tennis ball would pop up.



(Note: I answered this as if there was water in the beaker along with the steel balls and tennis ball. But the answer is not changed by my error.)



The term "van der Waals force" is sometimes used loosely for all intermolecular forces.






share|cite|improve this answer










New contributor




David Newell is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.






$endgroup$













  • $begingroup$
    Can't the steel balls float a table tennis ball even if they are very small?
    $endgroup$
    – enbin zheng
    2 days ago
















2












$begingroup$

Well, what if the steel balls were extremely small, say molecular size. In that case, the constraining annulus would look like a polished steel collar, and would likely hold down the ball even if the glass was shattered, underwater, in a swimming pool. . (The van der Waals forces, and metallic bonds, would account for that.)



But this example given, shows discreet balls of intermediate size, and unless they are magnetized, their coupling with the container is what allows restraint of the tennis ball.
If THIS setup was in the bottom of a deeper pool, and the beaker was shattered,
the steel balls would run radially away, and the tennis ball would pop up.



(Note: I answered this as if there was water in the beaker along with the steel balls and tennis ball. But the answer is not changed by my error.)



The term "van der Waals force" is sometimes used loosely for all intermolecular forces.






share|cite|improve this answer










New contributor




David Newell is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.






$endgroup$













  • $begingroup$
    Can't the steel balls float a table tennis ball even if they are very small?
    $endgroup$
    – enbin zheng
    2 days ago














2












2








2





$begingroup$

Well, what if the steel balls were extremely small, say molecular size. In that case, the constraining annulus would look like a polished steel collar, and would likely hold down the ball even if the glass was shattered, underwater, in a swimming pool. . (The van der Waals forces, and metallic bonds, would account for that.)



But this example given, shows discreet balls of intermediate size, and unless they are magnetized, their coupling with the container is what allows restraint of the tennis ball.
If THIS setup was in the bottom of a deeper pool, and the beaker was shattered,
the steel balls would run radially away, and the tennis ball would pop up.



(Note: I answered this as if there was water in the beaker along with the steel balls and tennis ball. But the answer is not changed by my error.)



The term "van der Waals force" is sometimes used loosely for all intermolecular forces.






share|cite|improve this answer










New contributor




David Newell is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.






$endgroup$



Well, what if the steel balls were extremely small, say molecular size. In that case, the constraining annulus would look like a polished steel collar, and would likely hold down the ball even if the glass was shattered, underwater, in a swimming pool. . (The van der Waals forces, and metallic bonds, would account for that.)



But this example given, shows discreet balls of intermediate size, and unless they are magnetized, their coupling with the container is what allows restraint of the tennis ball.
If THIS setup was in the bottom of a deeper pool, and the beaker was shattered,
the steel balls would run radially away, and the tennis ball would pop up.



(Note: I answered this as if there was water in the beaker along with the steel balls and tennis ball. But the answer is not changed by my error.)



The term "van der Waals force" is sometimes used loosely for all intermolecular forces.







share|cite|improve this answer










New contributor




David Newell is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.









share|cite|improve this answer



share|cite|improve this answer








edited 2 days ago





















New contributor




David Newell is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.









answered 2 days ago









David NewellDavid Newell

312




312




New contributor




David Newell is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.





New contributor





David Newell is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.






David Newell is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.












  • $begingroup$
    Can't the steel balls float a table tennis ball even if they are very small?
    $endgroup$
    – enbin zheng
    2 days ago


















  • $begingroup$
    Can't the steel balls float a table tennis ball even if they are very small?
    $endgroup$
    – enbin zheng
    2 days ago
















$begingroup$
Can't the steel balls float a table tennis ball even if they are very small?
$endgroup$
– enbin zheng
2 days ago




$begingroup$
Can't the steel balls float a table tennis ball even if they are very small?
$endgroup$
– enbin zheng
2 days ago


















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