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












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






newtonian-mechanics forces classical-mechanics fluid-dynamics







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    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






    newtonian-mechanics forces classical-mechanics fluid-dynamics







    share|cite|improve this question











    $endgroup$















      4












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      4


      1



      $begingroup$


      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






      newtonian-mechanics forces classical-mechanics fluid-dynamics







      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




      newtonian-mechanics forces classical-mechanics fluid-dynamics






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      edited 34 mins ago









      JAD

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      asked 4 hours ago









      enbin zhengenbin zheng

      445




      445






















          2 Answers
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          8












          $begingroup$

          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











          $endgroup$









          • 1




            $begingroup$
            -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
            1 hour ago








          • 2




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








          • 1




            $begingroup$
            @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
            1 hour ago



















          0












          $begingroup$

          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.






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            2 Answers
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            2 Answers
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            active

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            active

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            8












            $begingroup$

            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











            $endgroup$









            • 1




              $begingroup$
              -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
              1 hour ago








            • 2




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








            • 1




              $begingroup$
              @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
              1 hour ago
















            8












            $begingroup$

            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











            $endgroup$









            • 1




              $begingroup$
              -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
              1 hour ago








            • 2




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








            • 1




              $begingroup$
              @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
              1 hour ago














            8












            8








            8





            $begingroup$

            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 3 hours ago

























            answered 3 hours ago









            John RennieJohn Rennie

            279k44555801




            279k44555801








            • 1




              $begingroup$
              -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
              1 hour ago








            • 2




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








            • 1




              $begingroup$
              @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
              1 hour ago














            • 1




              $begingroup$
              -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
              1 hour ago








            • 2




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








            • 1




              $begingroup$
              @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
              1 hour ago








            1




            1




            $begingroup$
            -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
            1 hour 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
            1 hour ago






            2




            2




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






            1




            1




            $begingroup$
            @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
            1 hour 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
            1 hour ago











            0












            $begingroup$

            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.






            $endgroup$


















              0












              $begingroup$

              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.






              $endgroup$
















                0












                0








                0





                $begingroup$

                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 3 hours 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 3 hours ago









                David NewellDavid Newell

                112




                112




                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.






























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