Trjtdtk

Description list Formatting using enumitem

Two tailed t test for two companies' monthly profits

Can I set a Ready action to trigger when literally anything happens?

GFCI outlets - can they be repaired? Are they really needed at the end of a circuit?

Is it "common practice in Fourier transform spectroscopy to multiply the measured interferogram by an apodizing function"? If so, why?

Can someone clarify Hamming's notion of important problems in relation to modern academia?

How can saying a song's name be a copyright violation?

Does Dispel Magic work on Tiny Hut?

How do conventional missiles fly?

Was the Stack Exchange "Happy April Fools" page fitting with the '90's code?

Simple macro for new # symbol

Does Fukaya see all symplectic topology?

Where would I need my direct neural interface to be implanted?

Fair gambler's ruin problem intuition

Why would the Red Woman birth a shadow if she worshipped the Lord of the Light?

In Bayesian inference, why are some terms dropped from the posterior predictive?

Does the Idaho Potato Commission associate potato skins with healthy eating?

How to compactly explain secondary and tertiary characters without resorting to stereotypes?

files created then deleted at every second in tmp directory

What do you call someone who asks many questions?

What historical events would have to change in order to make 19th century "steampunk" technology possible?

Avoiding the "not like other girls" trope?

What's the meaning of "Sollensaussagen"?

Car headlights in a world without electricity

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

Why do the big nuts always remain at top? The Brazil-nut EffectPing-pong ball pontoonBackspin in outer space i.e. in a liquid with no viscosity: not changing direction?Terminal velocity of a steel ball in waterPhysics:Buoyant force and scale readingsIn table tennis, is it possible to play a ball in such a way that it does not bounce off the opponents half of the table?How to calculate minimum speed a ball needs to be rolling to move up when hitting a wall?Question about buoyancy force affecting mass measurementDoes an object need fluid under it to float?How high should be the edge of a billiard table to not allow undesirable pressures on balls when bouncing off?Pushing Two Balls Into One Another

15

5

$begingroup$

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?
Would it come up without friction?

newtonian-mechanics forces classical-mechanics fluid-dynamics

share|cite|improve this question

edited yesterday

enbin zheng

asked Mar 28 at 7:02

enbin zhengenbin zheng

11217

$endgroup$

• 
  
  
  

  
  10
  

  
  
  
  
  
  

  
  $begingroup$
  Shake the bowl a little. Like Brownian motion on water molecules.
  $endgroup$
  – Hot Licks
  Mar 28 at 19:31
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @HotLicks Would it come up without friction?
  $endgroup$
  – enbin zheng
  yesterday
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Some of the ball bearings sit above the midline of the ping-pong ball and push down on it. Even if the ball bearings too were free of all friction they would still be locked in place by the weight other ball bearings above them.
  $endgroup$
  – Hot Licks
  yesterday
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @HotLicks Because there is no friction, will the bearing balls at the bottom of the table tennis ball not produce buoyancy to the table tennis ball?
  $endgroup$
  – enbin zheng
  yesterday
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  They will produce buoyancy, but it's not a given that this will be sufficient to overcome the force of the balls pressing down from above, especially since the balls are apt to be "locked" in place due to being wedged together.
  $endgroup$
  – Hot Licks
  yesterday
  

  
  
  
  

 | 
show 2 more comments

15

5

$begingroup$

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?
Would it come up without friction?

newtonian-mechanics forces classical-mechanics fluid-dynamics

share|cite|improve this question

edited yesterday

enbin zheng

asked Mar 28 at 7:02

enbin zhengenbin zheng

11217

$endgroup$

• 
  
  
  

  
  10
  

  
  
  
  
  
  

  
  $begingroup$
  Shake the bowl a little. Like Brownian motion on water molecules.
  $endgroup$
  – Hot Licks
  Mar 28 at 19:31
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @HotLicks Would it come up without friction?
  $endgroup$
  – enbin zheng
  yesterday
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Some of the ball bearings sit above the midline of the ping-pong ball and push down on it. Even if the ball bearings too were free of all friction they would still be locked in place by the weight other ball bearings above them.
  $endgroup$
  – Hot Licks
  yesterday
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @HotLicks Because there is no friction, will the bearing balls at the bottom of the table tennis ball not produce buoyancy to the table tennis ball?
  $endgroup$
  – enbin zheng
  yesterday
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  They will produce buoyancy, but it's not a given that this will be sufficient to overcome the force of the balls pressing down from above, especially since the balls are apt to be "locked" in place due to being wedged together.
  $endgroup$
  – Hot Licks
  yesterday
  

  
  
  
  

 | 
show 2 more comments

$begingroup$

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?
Would it come up without friction?

newtonian-mechanics forces classical-mechanics fluid-dynamics

share|cite|improve this question

edited yesterday

enbin zheng

asked Mar 28 at 7:02

enbin zhengenbin zheng

11217

$endgroup$

share|cite|improve this question

edited yesterday

enbin zheng

asked Mar 28 at 7:02

enbin zhengenbin zheng

11217

share|cite|improve this question

edited yesterday

enbin zheng

asked Mar 28 at 7:02

enbin zhengenbin zheng

11217

asked Mar 28 at 7:02

enbin zhengenbin zheng

11217

asked Mar 28 at 7:02

enbin zhengenbin zheng

11217

2 Answers
2

active

oldest

votes

40

$begingroup$

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

edited Mar 28 at 8:03

answered Mar 28 at 7:55

John RennieJohn Rennie

279k44557804

$endgroup$

• 
  
  
  

  
  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
  Mar 28 at 9:48
  
  

  
  
  
  


• 
  
  
  

  
  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
  Mar 28 at 9:53
  
  

  
  
  
  


• 
  
  
  

  
  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
  Mar 28 at 10:20
  

  
  
  
  


• 
  
  
  

  
  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
  Mar 28 at 18:11
  

  
  
  
  


• 
  
  
  

  
  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
  Mar 28 at 18:17
  

  
  
  
  

 | 
show 7 more comments

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

edited Mar 28 at 7:59

answered Mar 28 at 7:49

David NewellDavid Newell

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.

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Can't the steel balls float a table tennis ball even if they are very small?
  $endgroup$
  – enbin zheng
  Mar 28 at 18:32
  

  
  
  
  

add a comment | 

Your Answer

StackExchange.ifUsing("editor", function ()
return StackExchange.using("mathjaxEditing", function ()
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix)
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
);
);
, "mathjax-editing");

StackExchange.ready(function()
var channelOptions =
tags: "".split(" "),
id: "151"
;
initTagRenderer("".split(" "), "".split(" "), channelOptions);

StackExchange.using("externalEditor", function()
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled)
StackExchange.using("snippets", function()
createEditor();
);

else
createEditor();

);

function createEditor()
StackExchange.prepareEditor(
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: false,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: null,
bindNavPrevention: true,
postfix: "",
imageUploader:
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
,
noCode: true, onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
);



);

draft saved

draft discarded

Sign up or log in

StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);

Sign up using Google

Sign up using Facebook

Sign up using Email and Password

Post as a guest

Name

Email

Required, but never shown

StackExchange.ready(
function ()
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fphysics.stackexchange.com%2fquestions%2f469105%2fwhy-doesnt-a-table-tennis-ball-float-on-a-surface-of-steel-balls-how-do-we-cal%23new-answer', 'question_page');

);

Post as a guest

Name

Email

Required, but never shown

40

$begingroup$

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

edited Mar 28 at 8:03

answered Mar 28 at 7:55

John RennieJohn Rennie

279k44557804

$endgroup$

• 
  
  
  

  
  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
  Mar 28 at 9:48
  
  

  
  
  
  


• 
  
  
  

  
  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
  Mar 28 at 9:53
  
  

  
  
  
  


• 
  
  
  

  
  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
  Mar 28 at 10:20
  

  
  
  
  


• 
  
  
  

  
  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
  Mar 28 at 18:11
  

  
  
  
  


• 
  
  
  

  
  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
  Mar 28 at 18:17
  

  
  
  
  

 | 
show 7 more comments

40

$begingroup$

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

edited Mar 28 at 8:03

answered Mar 28 at 7:55

John RennieJohn Rennie

279k44557804

$endgroup$

• 
  
  
  

  
  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
  Mar 28 at 9:48
  
  

  
  
  
  


• 
  
  
  

  
  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
  Mar 28 at 9:53
  
  

  
  
  
  


• 
  
  
  

  
  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
  Mar 28 at 10:20
  

  
  
  
  


• 
  
  
  

  
  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
  Mar 28 at 18:11
  

  
  
  
  


• 
  
  
  

  
  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
  Mar 28 at 18:17
  

  
  
  
  

 | 
show 7 more comments

$begingroup$

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

edited Mar 28 at 8:03

answered Mar 28 at 7:55

John RennieJohn Rennie

279k44557804

$endgroup$

share|cite|improve this answer

edited Mar 28 at 8:03

answered Mar 28 at 7:55

John RennieJohn Rennie

279k44557804

answered Mar 28 at 7:55

John RennieJohn Rennie

279k44557804

answered Mar 28 at 7:55

John RennieJohn Rennie

279k44557804

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

edited Mar 28 at 7:59

answered Mar 28 at 7:49

David NewellDavid Newell

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.

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Can't the steel balls float a table tennis ball even if they are very small?
  $endgroup$
  – enbin zheng
  Mar 28 at 18:32
  

  
  
  
  

add a comment | 

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

edited Mar 28 at 7:59

answered Mar 28 at 7:49

David NewellDavid Newell

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.

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Can't the steel balls float a table tennis ball even if they are very small?
  $endgroup$
  – enbin zheng
  Mar 28 at 18:32
  

  
  
  
  

add a comment | 

$begingroup$

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

edited Mar 28 at 7:59

answered Mar 28 at 7:49

David NewellDavid Newell

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.

$endgroup$

share|cite|improve this answer

edited Mar 28 at 7:59

answered Mar 28 at 7:49

David NewellDavid Newell

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.

answered Mar 28 at 7:49

David NewellDavid Newell

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.

answered Mar 28 at 7:49

David NewellDavid Newell

312