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  1. igorponger is offline
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    #1

    Picking up the ball under physical theory.



    I gathered a dozen of broken balls and measured the thickness on the outer shell. Balls with a thinner shell rotate faster.
    Table tennis is a wonderful sport where the laws of classical mechanics will govern all the things invariably, and I happened to study the subject as a young engineer ages ago. Thank you, my dear associate professor at the Department of the Structural Mechanics, you instructed me great lessons.

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    Last edited by igorponger; 12-03-2021 at 08:19 PM.

  2. Zwill is online now
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    #2
    I'm sorry but why? If the diameter and weight of the balls are roughly the same how can the wall thickness influence the rotation capability? I'm not a physicist but isn't the mass that is the limiting factor, and the grip of the plastic ball on the rubber?

    That being said the DHS made plastic balls fly faster than the seamless balls, and they feel harder too. DHS balls are easier and more fun to play with than seamless balls. DHS balls feel heavier on contact, but I'm not sure if they really arre heavier or just harder and this hardness is what's tricking the senses.


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    #3
    Quote Originally Posted by Zwill
    I'm sorry but why? If the diameter and weight of the balls are roughly the same how can the wall thickness influence the rotation capability? I'm not a physicist but isn't the mass that is the limiting factor, and the grip of the plastic ball on the rubber?

    That being said the DHS made plastic balls fly faster than the seamless balls, and they feel harder too. DHS balls are easier and more fun to play with than seamless balls. DHS balls feel heavier on contact, but I'm not sure if they really arre heavier or just harder and this hardness is what's tricking the senses.

    They do feel quite a bit heavier and thicker, I think they are actually heavier.

    Modestly, Leo

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    #4
    I gathered a dozen of broken balls and measured the thickness on the outer shell. Balls with a thinner shell rotate faster.
    They will if the radius of the balls is smaller. They will also rotate faster if the inertia of the thin shell balls is less.
    This assumes they are subject to the same stroke.
    The question is how good are your measurements.

    Table tennis is a wonderful sport where the laws of classical mechanics will govern all the things invariably, and I happened to study the subject as a young engineer ages ago. Thank you, my dear associate professor at the Department of the Structural Mechanics, you instructed me great lessons.
    The inertia formula for a hollow sphere assumes all the mass is in a microscopically small thin shell. If the shell is thicker and the radius is still the same then the inertia will be lower because not all the mass is at the distance of the radius.

    Plastic balls do seem harder. It is harder to dent them with your fingers and if you do they won't snap back like the old celluloid balls will.

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    #5
    Don’t forget that the ball with low inertia will loose it’s speed faster due to air resistance, in this case rotational…


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    L-zr
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    #6
    If the balls are of same size and weight, the one with the thinner shell will have greater inertia.

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    #7
    Quote Originally Posted by langel
    If the balls are of same size and weight, the one with the thinner shell will have greater inertia.
    No, same size and weight = same inertia.
    Steal a little and they throw You in jail, Steal a lot and they make You King... (Dylan)

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    #8

    Picking up the ball under physical theory.

    Quote Originally Posted by Lazer
    No, same size and weight = same inertia.
    Sorry, but no. M = F * d, with d being the distance between the center of the ball and the center of mass of the outer material, perpendicular.

    Consider three spherical objects of 40 mm diameter and 2.7g.

    1. a foam ball of uniform consistency
    2. a small, heavy spherical mass (diameter 2mm, 2.5g) centered by ultralight wires in an ultralight, ultrastrong otherwise hollow sphere
    3. a regular TT ball

    Do you really expect the same inertia? I’d say, 3>1>2.
    Last edited by yoass; 12-04-2021 at 02:09 PM.

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    #9
    Quote Originally Posted by Lazer
    No, same size and weight = same inertia.
    Langel is correct! The ball with the thinner shell has more rotational, we are talking about spin here, inertia than a thicker shell ball. This is because more of the mass is farther away from the center of the ball and the inertia goes up with the radius squared.

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    #10
    I think that ice skating spin would give a better visualization

    Have a nice spin!


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    #11
    Quote Originally Posted by yoass
    Sorry, but no. M = F * d, with d being the distance between the center of the ball and the center of mass of the outer material, perpendicular.

    Consider three spherical objects of 40 mm diameter and 2.7g.

    1. a foam ball of uniform consistency
    2. a small, heavy spherical mass (diameter 2mm, 2.5g) centered by ultralight wires in an ultralight, ultrastrong otherwise hollow sphere
    3. a regular TT ball

    Do you really expect the same inertia? I’d say, 3>1>2.
    I’d say You measure 😎
    Steal a little and they throw You in jail, Steal a lot and they make You King... (Dylan)

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    #12
    Quote Originally Posted by Lazer
    I’d say You measure 😎
    Then you calculate.
    Code:
    I=(2/5)*mass*((R^5-r^5)/(R^3-r^3))
    R is the outer radius of the ball. r is the inner radius of the hollow sphere. R-r is the thickness of the shell.
    The inertia I get larger as r approaches R. A hollow sphere with a shell of 0 thickness has an inertia
    Code:
    I = (2/3)*mass*R^2

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    #13
    No one noticed that according the math and physics, the thicker shell ball will have a lower inertia than a thin shell ball. This means the thicker shell ball should be easier to spin. This assumes the mass and outer diameter are the same and the amount of torque and duration of torque are the same.

    Assuming the inside radius of the thin shell ball is 20mm-0.25mm and the inside radius of the thick inside radius is 0.35mm, what is the difference in inertia. Is it enough to notice?

    This contradicts what igorponger said. Also, I find it disturbing that people "liked" something they didn't understand.

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    #14
    Quote Originally Posted by brokenball
    No one noticed that according the math and physics, the thicker shell ball will have a lower inertia than a thin shell ball. This means the thicker shell ball should be easier to spin. This assumes the mass and outer diameter are the same and the amount of torque and duration of torque are the same.

    Assuming the inside radius of the thin shell ball is 20mm-0.25mm and the inside radius of the thick inside radius is 0.35mm, what is the difference in inertia. Is it enough to notice?

    This contradicts what igorponger said. Also, I find it disturbing that people "liked" something they didn't understand.
    And this same inertia will let the ball rotate longer (as Lazer said) - and perhaps this is what Igor is observing...

    Perhaps - I don't know - the amount of spin you can give to a ball is not so much limited by the kinetic energy of the bat (there is plenty) but rather by the friction - then you in reality can't really make the ball with thicker shell rotate so much faster initially than the ball with thinner shell (of the same weight). And then, additionally, the ball with thinner shell will keep its rotation a bit longer. So perhaps he is not wrong...

    P.S. And, I mostly like exactly what I don't understand :-)
    P.P.S. Carl, don't worry. This time no sleep impairment. I promise.
    Last edited by latej; 12-08-2021 at 01:16 AM. Reason: Test

  15. Zwill is online now
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    #15
    IF we look at the ball like a flywheel it makes sense that the ball with the thicker wall would be able to accept more spin with the same force.

    But from actual play the DHS ball which in this case is the orange one winds up/stretches the rubber more. So I would wager that even with the same modern equipment, same stroke motion and speed the DHS ball would be more spiny because of the spring and stretch effect of the modern rubbers.
    But this should be measured, with so much unknown variables it's impractical to try to calculate it.

    It's a bit pointless to just talk about which ball can accept more spin on its own...

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    #16
    And this same inertia will let the ball rotate longer (as Lazer said) - and perhaps this is what Igor is observing...
    How much longer? Igor didn't mention how long. Spin a ball on a table. It will spin for a very long time and that is with the friction of the table to slow it down. The time the ball will spin is MUCH longer than the time it takes for the ball to go across the table.

    Didn't anybody do the math? This is what is wrong with TT forums. People just repeat the same old stuff without thinking.
    First the thin and thick shell TT balls have a difference in inertia of less than 0.5% assuming the masses and outside radii are the same. Can you honestly tell me you can tell the difference? This would be like a policeman giving you a ticket for going 60.4 in a 60 mile an hour zone based on observation. He must have calibrated eyeballs. Also Igor got it backwards

    Perhaps - I don't know..
    Agreed

    IF we look at the ball like a flywheel it makes sense that the ball with the thicker wall would be able to accept more spin with the same force
    What does it mean to accept more spin?

    But from actual play the DHS ball which in this case is the orange one winds up/stretches the rubber more.
    Why?

    But this should be measured, with so much unknown variables it's impractical to try to calculate it.
    There is only the mass, outside radius and inside radius. These can be measured easily. The formulas are above.

    It's a bit pointless to just talk about which ball can accept more spin on its own...
    I agree since "accepting more spin" doesn't make sense.

    Igor needs to tell us how he measured the spin of the ball. He doesn't have calibrated eyeballs. Measuring the spin that accurately with a high speed camera would be subject to more error. A ball rotating at 50 rev/s would take 40 frames per rev if the frames per second is 2000 FPS. 40 FPS would only be able to measure within 2.5%. I really doubt Igor's eye are that good.

    You have the equations for inertia of a sphere. You have NO excuses now.




  17. Zwill is online now
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    #17
    What does it mean to accept more spin?

    How much rotation a stroke can impart on a ball.


    Why?

    Why indeed? This is my question too? So far it seems it is totally pointless to discuss this topic since the difference is negligable and it should have next to no effect on the gameplay.
    But on the other hand there is a noticable difference between different make of balls. If they meet the specification then why is there a differance that is very noticable by humans? No one explained this.
    I only have a scale that can measure to a gram so I cannot measure if the balls really are 2.7g as they should be. If different makes have different weight then it could explain a lot.


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    #18
    Quote Originally Posted by Zwill
    How much rotation a stroke can impart on a ball.
    That is much different from how much spin a ball can accept. Have you ever seen a ball with a spin limit?

    My why question was asking you to justify your statement about DHS ball stretching the rubber more. More than what? You provide no facts to justify your statement.

    I only have a scale that can measure to a gram so I cannot measure if the balls really are 2.7g as they should be. If different makes have different weight then it could explain a lot.
    A hundredth of gram is much less than 0.5%. Even .02 grams is less than 1%.

    BTW, I do have a scale that measures down to 0.01 grams and it is calibrated every so often. It is used to count parts that go on circuit boards. Every once in a while I weigh a rubber, paddle or TT balls.

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    #19

    Have you ever seen a ball with a spin limit?

    Yes, I have in my entire life only seen balls with a spin limit. In fact every object I have seen has a spin limit. But not relevant to table tennis gameplay so why do you ask this question?

    My why question was asking you to justify your statement about DHS ball stretching the rubber more. More than what? You provide no facts to justify your statement.

    Actually in the 2nd post I was comparing DHS balls to seamless balls. I only have my senses to rely on as I don't have a speed gun, can't measure spin nor I have a scale that can weigh past gram accuracy. Still my question was not answered. Why does the DHS ball stretch the rubber more? No one answered otherwise either that it doesn't, but it would be an equally satisfying answer as well.

    A hundredth of gram is much less than 0.5%. Even .02 grams is less than 1%.

    I'm sorry, I fail to see the relevance. The difference in balls is 0.01-0.02g between brands? That is a very thight tolerance even between the same manufacturer. Can you confirm this?




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    #20
    Quote Originally Posted by brokenball
    My why question was asking you to justify your statement about DHS ball stretching the rubber more. More than what? You provide no facts to justify your statement.
    Â

    well if it's the same diameter and mass, but thinner, then it's gonna be more reluctant to spin, no?

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