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  1. edgesandnets is offline
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    #1

    Identifying the Magnus Effect in Table Tennis

    Wrote a new blog post introducing the Magnus Effect in table tennis to those who are unaware of it: https://edgesandnets.com/2021/04/23/identifying-the-magnus-effect-in-table-tennis/

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    Last edited by edgesandnets; 04-24-2021 at 02:40 AM.

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    #2
    Information about the Magnus effect is good if it is correct. I take issue with
    " When a spinning ball moves through a fluid (such as air), the ball “pushes” the air in one direction, and as a result of Newton’s Third Law (every action has an equal and opposite reaction), the air pushes back on the ball in the opposite direction"
    This is not a good explanation. There is difference in force between the top and the bottom. The air pressure is higher on the top of the ball than on the bottom of the ball so there is a net force downwards that accelerates the ball down wards. That is Newton's second law.
    There is a video of the fastest serve ever by a young Japanese player named Sakai.
    This serve would not be possible if it weren't for the Magnus effect. The Magnus effect is greater than the force on the ball due to gravity.

    The Magnus effect is proportional to the cross product of the spin x velocity. If the spin is at right angle to the trajectory of the ball then the spin is proportional the speed x velocity. Normally the speed and spin are a trade off. The pros have a much better touch and faster swing so they can increase either the speed or the spin or both.

    "we know from experience that even when the spin is light, a topspin ball will jump upward and forward "
    This is not right either. Top spin causes the ball to jump out faster and lower.

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    #3
    Quote Originally Posted by brokenball
    "we know from experience that even when the spin is light, a topspin ball will jump upward and forward "
    This is not right either. Top spin causes the ball to jump out faster and lower.

    Yes and no. A topspin ball's flight path bends down and so impact on the table is more vertical. More downward force on impact means more upward force on rebound. Higher bounce. It's true that the topspin itself causes the ball to jump forward (not upward) on impact, and get to the top of the bounce and dip down faster if there's enough residual topspin.

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  4. edgesandnets is offline
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    #4
    Quote Originally Posted by brokenball
    Information about the Magnus effect is good if it is correct. I take issue with
    " When a spinning ball moves through a fluid (such as air), the ball “pushes” the air in one direction, and as a result of Newton’s Third Law (every action has an equal and opposite reaction), the air pushes back on the ball in the opposite direction"
    This is not a good explanation. There is difference in force between the top and the bottom. The air pressure is higher on the top of the ball than on the bottom of the ball so there is a net force downwards that accelerates the ball down wards. That is Newton's second law.
    There is a video of the fastest serve ever by a young Japanese player named Sakai.
    This serve would not be possible if it weren't for the Magnus effect. The Magnus effect is greater than the force on the ball due to gravity.

    The Magnus effect is proportional to the cross product of the spin x velocity. If the spin is at right angle to the trajectory of the ball then the spin is proportional the speed x velocity. Normally the speed and spin are a trade off. The pros have a much better touch and faster swing so they can increase either the speed or the spin or both.

    "we know from experience that even when the spin is light, a topspin ball will jump upward and forward "
    This is not right either. Top spin causes the ball to jump out faster and lower.


    Thanks for your response! I've update parts of the post.

    1) The explanation I provided was similar to Wikipedia's "intuitive" explanation, and I thought it was easy to visualize, so I will leave it as is. I'm not great at physics, so I will defer to you what a more appropriate explanation would be.
    2) I was making the assumption (that I agree is not always correct) that the speed of the shot would more be limited by concern of the ball landing outside the table than limited raw strength from the player.
    3) I've updated the post to state that topspin is associated with bouncing upward and forward instead of causing the ball to jump upward and forward

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    #5
    Quote Originally Posted by Dr Evil

    Yes and no. A topspin ball's flight path bends down and so impact on the table is more vertical. More downward force on impact means more upward force on rebound. Higher bounce. It's true that the topspin itself causes the ball to jump forward (not upward) on impact, and get to the top of the bounce and dip down faster if there's enough residual topspin.

    Your logic sounds good but you forget that the ball still has top spin which makes it want to drop. On top of that the ball shoots out after the bounce if the tangential speed of the surface of the ball is faster than the speed of the ball. The ball leaves the table at a shallower angle.

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    #6
    Quote Originally Posted by edgesandnets
    Wrote a new blog post introducing the Magnus Effect in table tennis to those who are unaware of it: https://edgesandnets.com/2021/04/23/identifying-the-magnus-effect-in-table-tennis/

    Excuse me, why do you write an article on a matter you don't have the competence to deal with, addressed "to those who are unaware of" ?Brokenball made the very right corrections,and you have corrected,but at the same time you made the corrections just because " I'm not great at physics, so I will defer to you what a more appropriate explanation would be."Just imagine what would be the result if you were corrected not by the competency of Brokenball, but of some one else, even more incompetent than youIf that was just a forum thread, than OK, its a matter of discussion.But you write an article for a public net consumption, expected to be believed as true by all readers.Sorry for the sharp and negative response, but You have to be more responsible when you try to teach the others.

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    #7
    Quote Originally Posted by brokenball
    Your logic sounds good but you forget that the ball still has top spin which makes it want to drop. On top of that the ball shoots out after the bounce if the tangential speed of the surface of the ball is faster than the speed of the ball. The ball leaves the table at a shallower angle.

    Interesting contention. You may be right (I haven't done the math) but I'm skeptical. For argument's sake I'll assume you're saying that more topspin always causes the ball to leave the table at a shallower angle, i.e., it doesn't bounce as high. Consider two balls with equal energy, a loop (more topspin, less speed) and a drive (less topspin, more speed). You say the drive always bounces higher than the loop. In other words, after the loop bounces, the increased upward force of the more vertical impact is always dominated by the increased downward force of the Magnus effect. But doesn't this depend on trajectory? So for example a sufficiently curved loop will bounce higher than a sufficiently flat drive of equal energy. Is this wrong? Maybe you can't get sufficiently curved vs sufficiently flat with the same energy. Have you done the math?


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    #8

    ZIGZAG BALLS

    This is the most amazing aerodynamical effect one can get on some occasions when playing table tennis with some specific strokes. Moreover, wobbling (Zigzag) balls can be generated manually on purpose, with the help of a physical theory of zigzag balls as worked out by a Russian jet-missile engineer, a guy really keen on the game.
    Be happy.


    https://sun9-53.userapi.com/TwEjiBIc...rtY4IbxHJg.jpg




    Reason:
    Error: File upload aborted.
    What hell?? Where is the tumb image?
    Last edited by igorponger; 04-24-2021 at 07:09 PM. Reason: Error: File upload aborted. What hell????

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    #9
    Quote Originally Posted by Dr Evil

    Interesting contention. You may be right (I haven't done the math) but I'm skeptical. For argument's sake I'll assume you're saying that more topspin always causes the ball to leave the table at a shallower angle, i.e., it doesn't bounce as high.

    I might but there is the Magnus effect, coefficient of restitution, and air friction.
    Consider two balls with equal energy, a loop (more topspin, less speed) and a drive (less topspin, more speed). You say the drive always bounces higher than the loop.
    Given the same angle of incidence, yes.
    In other words, after the loop bounces, the increased upward force of the more vertical impact is always dominated by the increased downward force of the Magnus effect.
    The ball will only bounce up at about 90% of its impact speed if there is no spin.
    But doesn't this depend on trajectory? So for example a sufficiently curved loop will bounce higher than a sufficiently flat drive of equal energy. Is this wrong?
    A lot depends on the maximum height of the trajectory. Optimal is when this occurs just as the ball passes over the net and barely passes over the net.
    Maybe you can't get sufficiently curved vs sufficiently flat with the same energy. Have you done the math?

    I have on the flight of the ball up till the time it hits the table but not afterwards because I didn't have a good model for the effects of the impact. I did the math for the Sakai fastest server ever years ago. I examined the video frame by frame and if I remember right, it took about 0.24 seconds for the ball to go from first impact on the server's side to the impact on the other side. The top spin that Sakai imparted on the ball did not make if bounce high. If it did bounce high the serve wouldn't work.

    So a question for you. If you just use your hand to toss a TT ball without spin so it barely goes over the net and then land's on the other side, how long does that take?
    You may ignore air resistance. Your answer will be close enough. The net is 165.25 cm high. Assume the bottom of the ball reaches a height of 16.3 cm to barely go over the net.
    Edit, my first coach and I would have a warm up drill where we would loop and counter loop in the effort for the other person to miss by the ball going below the other person's paddle.
    Last edited by brokenball; 04-24-2021 at 07:48 PM.

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    #10
    Hi BB,

    Regarding the axis of spin, initially is this set by the bat angle and orientation of the bat at the point of contact?
    Does the axis change as the ball travels through the air? (Regarding balls with side and top or side and backspin rather than those with ‘pure’ top or backspin which should maintain the same axis of spin throughout the balls flight?)

    cheers,


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    #11
    Quote Originally Posted by brokenball
    Dr Evil
    Consider two balls with equal energy, a loop (more topspin, less speed) and a drive (less topspin, more speed). You say the drive always bounces higher than the loop.

    brokenball
    Given the same angle of incidence, yes.

    No argument there. But the point is the loop usually has a steeper angle of incidence because of the extra topspin. So -- unless the Magnus effect of the post-bounce topspin always dominates the extra upward force of the more vertical impact -- the loop can bounce higher. Sure seems to me like slower curvier loops tend to bounce higher than faster flatter drives. Could be wrong, could be they're slower and tend to land shorter on the table and so have more time to reach the top of the bounce, or some other factor. But I'm still skeptical. On the other hand, I have no doubt that flat trajectory heavy topspin loops (and fast serves) stay low and dip lower. Clearly in that case, because of the shallow angle of incidence, the Magnus effect dominates post-bounce.

    *
    broken ball
    So a question for you. If you just use your hand to toss a TT ball without spin so it barely goes over the net and then land's on the other side, how long does that take? You may ignore air resistance. Your answer will be close enough. The net is 165.25 cm high. Assume the bottom of the ball reaches a height of 16.3 cm to barely go over the net.
    *

    Not sure the point of your question. Free fall time from net height without air resistance is a little under a fifth of a second. If I hand toss a ball so it reaches maximum height as it skims over the top of the net, time of flight will depend on how far away from the net I was standing when I tossed the ball. However, the height of the bounce won't change depending on distance of flight. Maybe that's your point. But height of bounce will change depending on maximum height of the trajectory.

    Edit: Okay, I think I get your point. You're saying that if the loop and the drive have the same maximum height over the table, the loop (with more topspin) will bounce lower. That I agree with. But if they have the same energy, the loop will usually be loopier (higher maximum height) and so will bounce higher unless the post-bounce Magnus effect dominates.

    Last edited by Dr Evil; 04-24-2021 at 10:34 PM.

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    #12
    Quote Originally Posted by IB66
    Hi BB,

    Regarding the axis of spin, initially is this set by the bat angle and orientation of the bat at the point of contact?
    Yes, but also the direction of the brushing motion. This is very important because the ball can be hit at the same place but get different spin depending on the brushing ( tangential ) motion of the paddle relative to the ball.

    Does the axis change as the ball travels through the air? (Regarding balls with side and top or side and backspin rather than those with ‘pure’ top or backspin which should maintain the same axis of spin throughout the balls flight?)
    No, I haven't seen any evidence that change the axis of spin. That would take an external force other than the air.

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    #13
    Quote Originally Posted by Dr Evil
    [p]No argument there. But the point is the loop usually has a steeper angle of incidence because of the extra topspin.
    This is an assumption. I do agree you are right in some cases but not all. I like to make slow spinny loops at angles and there must be more arc to land the ball on the table near the side edge. This is an example of where the angle of incidence may be higher with a loop but then this shot is not even possible with a flat hit. I think it is better to compare long balls that bounce close to the edge of the table because this is possible with both flat hits and loops but the loops can be hit from a lower position whereas a flat hit must have a line of sight and the ball is actually hit into the table as opposed to a good loop where the ball is dropping only due to gravity and the Magnus effect.

    So -- unless the Magnus effect of the post-bounce topspin always dominates the extra upward force of the more vertical impact -- the loop can bounce higher.
    This is why I bring up the Sakai video. The Magnus effect does dominate but that is an extreme case.

    Sure seems to me like slower curvier loops tend to bounce higher than faster flatter drives. Could be wrong, could be they're slower and tend to land shorter on the table and so have more time to reach the top of the bounce, or some other factor.
    From my experience, this is not the case. I have played choppers where I must match the spin of the ball or add some to be more aggressive but since I am not a pro, the ball go over the net without much speed but they do drop causing the chopper to move forward to get to the ball.

    I will try to find a more definitive answer.

    But I'm still skeptical.
    By all means be as skeptical as you want. I encourage it. I have always been skeptical and questioned everything. Your question is good but we need an apples to apples comparison.

    I bring up looping and flat hits. Obviously there are an infinite number of variations in between.


    On the other hand, I have no doubt that flat trajectory heavy topspin loops (and fast serves) stay low and dip lower. Clearly in that case, because of the shallow angle of incidence, the Magnus effect dominates post-bounce.
    Yes.


    broken ball
    "So a question for you. If you just use your hand to toss a TT ball without spin so it barely goes over the net and then land's on the other side, how long does that take? You may ignore air resistance. Your answer will be close enough. The net is 165.25 cm high. Assume the bottom of the ball reaches a height of 16.3 cm to barely go over the net."

    Not sure the point of your question. Free fall time from net height without air resistance is a little under a fifth of a second. If I hand toss a ball so it reaches maximum height as it skims over the top of the net, time of flight will depend on how far away from the net I was standing when I tossed the ball. However, the height of the bounce won't change depending on distance of flight. Maybe that's your point. But height of bounce will change depending on maximum height of the trajectory.
    So how does Sakai serve a ball that goes from his end of the table to the other end of the table in about 0.24 seconds?
    How does the ball bounce over the net and come down in a that time? Magnus effect.

    Edit: Okay, I think I get your point. You're saying that if the loop and the drive have the same maximum height over the table, the loop (with more topspin) will bounce lower. That I agree with. But if they have the same energy, the loop will usually be loopier (higher maximum height) and so will bounce higher unless the post-bounce Magnus effect dominates.
    Wait let me work on this for the best answer. I know from experience that looped balls bounce out low and fast. I either need more people to verify this or I need to model it.

    Good questions.

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    #14
    Quote Originally Posted by langel

    But you write an article for a public net consumption, expected to be believed as true by all readers.Sorry for the sharp and negative response, but You have to be more responsible when you try to teach the others.

    Could not agree more: I've been writing for local newspapers before the ineternet era, as I was considered being a "music expert", the chiefs editors I had to deal with were very clear with me: finding sources isn't the only sufficient matter, you have to 1- verify them 2- understand them 3- rendering/making them inteligible/understandable. If one is missing, the others are comprommised. You did not understand the source, then you were not able to make it inteligible.I think this is very serious. Intellectual integrity should matter first way before searching for fame. That's the problem nowadays with internets and social medias: hierarchical information does not exist anymore, anyone can write BS and be considered an expert, see what's happenning nowadays with rejection of science about the pandemic and vaccines.

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    Last edited by Music&Ping; 04-25-2021 at 03:37 AM.

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    #15
    Quote Originally Posted by brokenball
    Yes, but also the direction of the brushing motion. This is very important because the ball can be hit at the same place but get different spin depending on the brushing ( tangential ) motion of the paddle relative to the ball.


    No, I haven't seen any evidence that change the axis of spin. That would take an external force other than the air.
    Cheers BB,

    So the bounce could have an effect? Which could also be affected by the tables surface finish? Skid or grip ?

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    #16
    Notice that mytt is clueless on this topic. Where are their PhDs and engineers?
    http://mytabletennis.net/forum/topic...1.html#1111961

    "That's the problem nowadays with internets and social medias: hierarchical information does not exist anymore, anyone can write BS and be considered an expert"
    This is sad. This is why I question everything. You should always know the credentials of those "telling you the truth"
    So who am I? Even Yogi knows I am a "hard headed engineer" but i am more than that.
    BTW, I write magazine articles for many engineering publications and I have been doing it for about 20 years. If I were a fake people would have called BS on what I write.


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    #17
    Bernoulli's principle alone is not sufficient to explain the Magnus effect, as it doesn't account for the negative Magnus effect and lift/drag crisis that could happen under specific conditions. Flow separation, AKA boundary layer separation, that involves the boundary layer, point of separation, laminar-turbulent transition, wake, lift, drag, Reynolds number and spin parameter etc. is necessary to better explain the Magnus effect.

    To keep it short, boundary layer is a thin layer of air that attaches to the surface of the ball in the direction of travel as the ball flies across the air. Depending on the spin, the boundary layer could separate at different points on the forward-moving and backward-moving sides and transition from laminar flow to turbulent flow, which is determined by the Reynolds number and spin parameter. The asymmetrical flow separation deflects the air and creates a pressure difference in the form of wake trailing behind the ball, generating an aerodynamic force that is comprised of lift and drag, in addition to the friction between the surface of the ball and the air.

    Contrary to the results and predictions from wind tunnel tests, a few Japanese studies show that the negative Magnus effect that is common in some sports like football, volleyball, and baseball, where a spinning ball exihibits unpredictable behaviors in flight(e.g., a backspin ball curves down like how a topspin ball does), doesn't occur in free-flight tests of an actual table tennis ball launched with backspin from a customized 3-rotor robot within the range of speed and spin typically seen in table tennis. However, they find that there is a significant drop in the lift coefficient for some combinations of Reynolds number and spin parameter where the negative Magnus force is observed in wind tunnel and water channel tests. This "lift crisis" causes the lift force exerted on the ball to fluctuate widely due to the unsteady laminar-turbulent transition. A similar but less prominent trend is also observed for the drag coefficient.
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    #18
    Quote Originally Posted by zeio
    Bernoulli's principle alone is not sufficient to explain the Magnus effect, as it doesn't account for the negative Magnus effect and lift/drag crisis that could happen under specific conditions. Flow separation, AKA boundary layer separation, that involves the boundary layer, point of separation, laminar-turbulent transition, wake, lift, drag, Reynolds number and spin parameter etc. is necessary to better explain the Magnus effect.
    No one mentioned the Bernoulli principle. The formulas are quite different.

    To keep it short, boundary layer is a thin layer of air that attaches to the surface of the ball in the direction of travel as the ball flies across the air. Depending on the spin, the boundary layer could separate at different points on the forward-moving and backward-moving sides and transition from laminar flow to turbulent flow, which is determined by the Reynolds number and spin parameter. The asymmetrical flow separation deflects the air and creates a pressure difference in the form of wake trailing behind the ball, generating an aerodynamic force that is comprised of lift and drag, in addition to the friction between the surface of the ball and the air.
    Hence dimples on golf ball. If you watch the better drivers at the range you can see the angle of the trajectory of the ball often increases due to back spin and the dimples. TV golf programs show this effect a lot because the pros are driving the ball.

    How fast must a TT ball go to have turbulent flow so it wobbles? How much is the wobble? What is the frequency of the wobble. Can you even notice the wobble if the ball is going that fast.

    Contrary to the results and predictions from wind tunnel tests, a few Japanese studies show that the negative Magnus effect that is common in some sports like football, volleyball, and baseball, where a spinning ball exihibits unpredictable behaviors in flight(e.g., a backspin ball curves down like how a topspin ball does), doesn't occur in free-flight tests of an actual table tennis ball launched with backspin from a customized 3-rotor robot within the range of speed and spin typically seen in table tennis. However, they find that there is a significant drop in the lift coefficient for some combinations of Reynolds number and spin parameter where the negative Magnus force is observed in wind tunnel and water channel tests. This "lift crisis" causes the lift force exerted on the ball to fluctuate widely due to the unsteady laminar-turbulent transition. A similar but less prominent trend is also observed for the drag coefficient.
    Could this be like applying reduced mass to TT impacts?
    I would like to see the studies. I don't pay much attention to studies that can't be repeated and even those like cold fusion which I knew was a hoax from the start. Do you remember that BS?
    If there is a a "lift crises" then what cause it?
    Are we just lucky that the TT balls land on the table?

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    #19
    Not another joke. Talk about lecturing others while being clueless yourself. Why is the air pressure higher on one side? Why is there a non-zero net force acting on the ball? Those effectively imply the Bernoulli's principle. But like the Newton's 3rd law of motion, that is insufficient to explain the lift. In fact, both of them together are insufficient. For those interested, check out the links below to find out why. They may be about airplanes, but the same principle of lift applies for the Magnus effect.

    https://www.scientificamerican.com/a...ay-in-the-air/
    https://en.wikipedia.org/wiki/Lift_(force)#Bluff_bodies
    https://en.wikipedia.org/wiki/Lift_(...ons_fall_short

    At the end of the day, take it or leave it. I have better things to do than waste my time translating those studies for jerks. May as well post this meme since this will be my only reply.

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    Senior TTD Member 533 155

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    Jan 2012
    Former Soviet Union
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    #20
    BTW The truth is that Magnus in reality observed cilindrical objects, not spheres, Heh-heh-heh.

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