conservation of angular momentum, whip physics, and a serve question

Discussion in 'Tennis Tips/Instruction' started by spacediver, Aug 29, 2010.

  1. spacediver

    spacediver Hall of Fame

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    understood :)

    when you paste it in the textbox on this messageboard, just manually delete the font tags. That should fix the problem.
     
  2. toly

    toly Hall of Fame

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    I'm trying to be more specific. I described the wrist behavior during last moment (pronation phase) of the kick serve. Yes, I copied the part of my article to save my time. What particular question do you have about my text?
     
  3. spacediver

    spacediver Hall of Fame

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    I'm just not sure where you're going.

    You asked me a question about whip physics as it relates to the kick serve, and I answered you, and then you suddenly paste this other stuff. I'm not sure how to respond.

    First off, did you understand my answer? Did it help? Do you agree or disagree with my reasoning? Do you understand the significance of the parallel circles and how that relates to momentum transfer? Do you understand how pronating the forearm changes the way in which the wrist reacts to momentum transfer?
     
    Last edited: Sep 25, 2010
  4. toly

    toly Hall of Fame

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    I can answer on your post step by step only, because there too much bits and pieces to discuss. This is about kick serve. We are trying to use Whip Effect to increase the racquet velocity. We want to use our arm like speed multiplier. But, our main concern is ball velocity. In case of the kick serve, the wrist cannot contribute anything to the ball velocity, because it is in stable position (except ulnar deviation). Ulnar deviation produces just spin. The main contributor to the ball velocity would be pronation. But, pronation mostly generated by shoulder joint (handle). Angular speed of the pronation of the upper arm is equal to the angular speed of the wrist. It means efficiency of the Whip Effect =0. There is no Whip Effect at all.
     
  5. spacediver

    spacediver Hall of Fame

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    If by whip effect you mean channeling of angular momentum, then there certainly is such an effect in the kick serve.

    I've explained this over and over, but it's hard to communicate with you since you don't ask for clarification when you don't understand.

    Do you agree that regardless of serve type (flat, kick, whatever), that the momentum of the trunk gets channeled into the arm? And if so, do you see how trunk rotation can cause the elbow to extend?

    Do you even understand what I mean by elbow extension?
     
  6. spacediver

    spacediver Hall of Fame

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    I agree that in the kick serve, wrist action probably doesn't contribute much, if anything, towards generating spin on the ball. But elbow extension certainly does.
     
  7. toly

    toly Hall of Fame

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    Give me some prove please. I don't want speculation.
    If you do not agree that efficiency of the Whip Effect = 0. Prove it please. I do not understand expressions like "contribute much". Please be more specific. Enough quality, give me please some quantity.
    What is elbow extension?
     
  8. spacediver

    spacediver Hall of Fame

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    this is elbow extension:

    [​IMG]

    Do you now understand how this elbow joint rotation contributes towards the serve?
     
  9. toly

    toly Hall of Fame

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    Now, you are going to include trunk motion in our discussion. You are also talking about legs and so on. In this case you should describe Kinetic Chain. It looks you have to define new handle (trunk, or legs, or so on) of our passive whip model.
     
  10. toly

    toly Hall of Fame

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    The wrist ulnar deviation does. I stated it many times.
     
  11. toly

    toly Hall of Fame

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  12. spacediver

    spacediver Hall of Fame

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    Toly, I hope you don't take this the wrong way, but I feel I've explained myself many times. I suggest you go back through this thread, and read my posts carefully.

    Also, I think it would be helpful if you stopped getting caught up with the whip analogy and thinking of it in terms of one handle and one strap.

    Think of it as a system that generates and transfers momentum.
     
  13. spacediver

    spacediver Hall of Fame

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    sorry, i meant wrist flexion.
     
  14. toly

    toly Hall of Fame

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    I’m sorry. This all staff is very complicated for me. I’m just trying to simplify the matter as much as possible. This is my simple question. Suppose I have two strait pieces of wood, and I connected them end by end, but allow them to rotate with 360 degrees range without friction. There is no gravity. The angle between them is 90 degrees. I move them along the straight line with speed V. There are two linear momentums: M1xV and M2xV. I stop the first one. How many percents of the first stick energy will be transferred to the second one?
     
  15. toly

    toly Hall of Fame

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    About speen

    The wrist ulnar deviation can be much faster motion then elbow extension. That’s why, it should be used as main contributor (not elbow)to the spin.
     
  16. toly

    toly Hall of Fame

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    According to my calculation, the pronation can generate angular speed around ΩH=6000°/sec. Legs (knees bend, jump, or whatever), shoulders seesaw movement, trunk rotation, and fast elbow joint with forearm, all together produce just ΩV=600°/sec. It would be much more useful to discuss first the arm motion, because it can be the main contributor to the any tennis stroke.
     
  17. spacediver

    spacediver Hall of Fame

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    under "ideal" conditions, my guess would be 100% transfer.
     
  18. spacediver

    spacediver Hall of Fame

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    But you agree that if ulnar deviation plays a role in the kick serve, then the power used to deviate the wrist is going to be channeled from elbow extension, right?
     
  19. spacediver

    spacediver Hall of Fame

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    You're forgetting to factor in the way in which a joint rotation can contribute to useful racquet head speed, and to do this, you need to factor in the direction that the racquet head moves in response to a particular joint rotation, and the leverage involved.

    In the case of elbow pronation, there is not much mechanical leverage relative to trunk rotation. So even though the trunk can be rotating at a slower angular speed relative to forearm pronation, the trunk rotation leads to much greater racquet head speed.
     
  20. spacediver

    spacediver Hall of Fame

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    as an example of leverage as it relates to forearm pronation, consider the following:

    if a racquet is twisting along its longitudinal axis (very similar to what happens during forearm pronation), then this rotation will provide no useful racquet head speed if the ball is hit in the center of the racquet face, even if the racquet is spinning at a million miles an hour. However, if the ball is struck on the side edge of the racquet head (near frame of racquet at 3 or 9 o'clock), then the ball will absorb momentum from the racquet.
     
    Last edited: Sep 27, 2010
  21. callmethedoctor

    callmethedoctor Banned

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    Very interesting thread. My input (and I didn't read the whole thread) would be that it depends on whether the muscles that would be used in an "armed" serve (triceps, pectoralis, forearm) can accelerate as fast as this whip effect. And that question leads to another- whether the arm can create a whip effect. Is the forearm significantly lighter than the upper arm? Is the racket lighter than the forearm?
    Furthermore, I believe the serve is the only stroke this can apply to because trying to "whip" on, say, a forehand, isn't going to work because unless you consciously pronate, the elbow doesn't bend that way, and even if you do, the wrist won't cooperate either. I think this "whip physics" could be the reason people tell you to serve loosely. Then again, if you do a whip type serve, then the racket loses significant velocity as it contacts the ball, maybe leaving the ball with much less momentum despite the possible increase in racket head velocity.
    I would also say the handle has to be the torso. You can't lift your racket without bending the torso.
    I believe the best way to do this is simple experiment.
     
  22. toly

    toly Hall of Fame

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    I believe, it could be completely different result. First I should think about the expression “I stop the first stick". What way I can do it? Imagine, it hits tennis net post. This action produces some damage to the post and we lose some stick’s kinetic energy. The stick also gets some damage, again some lost of energy. Then post makes stick to bounce back. I want to prevent this from happening. By using my hand I press the stick back to the post. The stick hits my hand and also creates some damage to it. I push stick back to the post and so on. All this process is harmful for all participants and energy consuming. I believe the stick can lose all its kinetic energy during this struggle. I assume, the joint between sticks allows the second one rotates in the plane perpendicular to the first stick. One end of the second stick abruptly stopped. There also would be damaging and energy consuming fight between joint and second stick, but not so dramatic. Because, the second stick has option, to convert its linear energy to angular one. This energy would be equal m2xVxV/2 minus some lost of energy, because of the struggle with joint and first stick. The second stick without friction will rotate forever. So far, I do not see any energy transfer from first stick to the second one, but everybody got hurt. Right now I understand stuff like tennis elbow, wrist and shoulder injuries. That why Federer never straightens his elbow. He is really smart guy.
     
  23. spacediver

    spacediver Hall of Fame

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    Yes these are important factors. The amount of passive vs active torque generated at each joint may vary from individual to individual depending on an individual's muscular power potential at each joint, the proportions and weight of their limbs and racquet.

    Well the channeling of momentum can occur between torso and arms, where the torso is the heavier portion and the arm the lighter one.


    I don't follow your logic here.
     
  24. toly

    toly Hall of Fame

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    I agree, except, you have to keep boll near 3 o’clock. If it’s near 9 o’clock, the racquet runs away from the ball. But, in case of the flat serve you can completely lose the ball direction control. The pros never use this idea for flat serve. In case of the slice serve pros usually keep ball near 9 o’clock and during impact they just brush it in 3 o’clock direction. They are using the same approach to any spin serve. To create good efficiency of the pronation (it is = sin(beta)) you should provide proper value of the angle beta. The angle beta is angle between long axis of the racket and forearm. I describe this matter in details in my first post on this site. I already recommended you to read this post. You also can watch the videos: http://www.vimeo.com/10566621 http://www.youtube.com/watch?v=1t6bL...eature=related[/
     
  25. toly

    toly Hall of Fame

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    Usually the racket’s weight is much more than the ball’s weight. That’s why there is no problem. The racket speed practically is not affected by the ball.
     
  26. spacediver

    spacediver Hall of Fame

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    I think you may have misunderstood what I meant by using the clock analogy.

    You seem to be using 3 and 9 o'clock to describe the location on the ball. I'm using it with reference to the racquet face.

    In the image below, 3 o'clock is the red patch, 9 o'clock is the blue patch, and the center is green. No matter how fast the racquet is spinning along its longitudinal axis (the purple line), if the ball is struck in the green area, no momentum will be transfered to the ball.

    [​IMG]
     
  27. callmethedoctor

    callmethedoctor Banned

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    Sorry, should've explained more. Momentum= velocity*mass, right? Well, for me at least, the racket is only 5 times heavier than the ball. Therefore, it can be theorized that 20% of the velocity of the racket gets transferred in order to make the ball move. Or, I could be totally just making this up. Idk, just hypothesizing to try to find a reason. I mean, if it didn't affect the velocity, you wouldn't even know you were hitting the ball.
     
  28. J011yroger

    J011yroger G.O.A.T.

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    A racquet should never rotate along its longitudinal axis. During forearm pronation on a serve the racquet should be close to perpendicular to the forearm.

    J
     
  29. toly

    toly Hall of Fame

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    I agree about green patch. The linear speed of the any racquet patch is |V|= Ω x R, where: Ω - angular racquet speed, R - rotation radius. R is the shortest distance between the long axis of the racquet and particular patch. In case of the green patch R=0, hence |V|=0.

    R(blue) = R(red), that’s why they have the same speed which is not equal to zero. But, they have opposite directions of the corresponding linear velocities (V). When one patch (for example, blue) moves into ball, second one (red) inevitably moves in opposite direction (away from the ball).
     
  30. toly

    toly Hall of Fame

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    I’m absolutely agreed with you. But, when I analyze frame by frame 300 fps video, I do not see any racquet deceleration. Hence, something should compensate the inevitable lose of the kinetic energy. What it can be? I think, arm muscles (including wrist muscles) provide this compensation. This is one more argument that wrist cannot be passive. But, I’m not 100% sure about it.
     
  31. toly

    toly Hall of Fame

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    I agree, that racquet should never rotate along its longitudinal axis, because in this case radius of the rotation could be zero (if ball is on green patch it is exactly zero) and pronation component of the racquet speed will be zero. But, it is extreme that during forearm pronation on a serve the racquet should be close to perpendicular (angle beta=90) to the axis of the forearm. The best servers usually keep angle beta around 30-45. If the angle beta = 90, pronation efficiency =100%.
     
  32. spacediver

    spacediver Hall of Fame

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    agreed. I was just using that to illustrate the concept of how you need to factor in leverage when talking about joint rotations.

    Some players adopt grips that makes the racquet twist close to this fashion right before contact though (and as a result lose valuable leverage).
     
  33. J011yroger

    J011yroger G.O.A.T.

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    I find it easier to just not read or reply to toly's posts.

    But to each his own lol.

    J
     
  34. callmethedoctor

    callmethedoctor Banned

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    Well of course some muscle has to provide the force compensation if there is no loss of energy. The question is whether this muscle force is worth the /possible/ lost speed by not using this whip motion. Maybe we should whip until the contact point for max acceleration, and from there push with our muscles? Is this even possible to time correctly? Maybe. Will this provide greater acceleration? Maybe. I would try it out for you, but my leg is broken and isn't getting better any time soon.
     
  35. julian

    julian Hall of Fame

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    Which time moment?

    Which time moment are talking about?
    See

    http://www.youtube.com/watch?v=sfA3RlAXpdg&feature=sub

    Do you talk before a contact or AFTER a contact
     
    Last edited: Sep 28, 2010
  36. J011yroger

    J011yroger G.O.A.T.

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  37. 5263

    5263 G.O.A.T.

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    The guy in the vid is making ref to the serve at Miami, but wasn't the talk all about how the serve speed jumped up at the US OPen? well after Miami?
    Maybe I missed something.

    On the other point, not having read a lot of the past post, the accel thru the contact should not show the racket slowing at impact (except a volley maybe), but actually accelerating right thru it due to energy expended during that phase.
     
  38. J011yroger

    J011yroger G.O.A.T.

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    The guy making the video pauses the video at different times to make his point look valid.

    J
     
  39. toly

    toly Hall of Fame

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    Energy Transfer

    Let me analyze one more very simple example. I’m driving car. The car speed is V. My speed is also V. Then the car collides with mountain. Will the car transfer any its kinetic energy to me? If it did I would be definitely killed, even save belt doesn’t save me. Thanks god, the car just cannot do it. My speed still be equal V. I believe the same result would be in case any tennis shot. Our body just cannot transfer kinetic energy of the trunk, knees, shoulder (whatever) to the wrist. Otherwise, our wrist would be completely destroyed. What do you think about this matter?
    How do you insert pictures in your post? Thank you very much for deleting font tags instructions. It really helps me.
     
  40. toly

    toly Hall of Fame

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    Energy Compensation

    Let’s analyze the example. The ball (during impact) stays on the racquet bed around 2 msec. Assume, it leave racquet with speed equal 125 mph (200 km/h). The mass of the ball is 57 grams. I calculated kinetic energy of the ball and divided it by time = 2 msec. It would be the power of the racquet produced during impact and it is 60 hp. The result is astonishing! Assume the racquet lost 20% of its the kinetic energy. To compensate it, the arm muscles (or just wrist muscles???) should generate power = 12hp. I believe it is completely impossible. In this case, it doesn’t matter the wrist is active or not. Maybe my analyses of the 300 fps were wrong? To make it accurate I need at list 1000 fps videos. But, I cannot find them.

     
  41. spacediver

    spacediver Hall of Fame

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    I'm not sure how the car analogy maps onto this question, so I'll just answer this part of the question.

    My understanding is that your joints can absorb a lot more force if they are loose.

    Secondly, not all the force is being channeled through the wrist. At the moment of impact, wrist flexion contributes 24% of racquet speed (according to Brian's data). The major contributor is elbow extension (35%).

    Thirdly, a proper follow through can prevent the joints from being overloaded. For example, right after contact, the elbow starts to flex. This, combined with the shoulder internal rotation, "decreases the magnitude of the bone on bone contact at full (elbow) extension" (from one of Brian's articles).

    Use the [ img ] [ /img ] tags (insert the url of the image in between the tags)

    glad to help :)
     
  42. toly

    toly Hall of Fame

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    Url

    Where I can get url, if I keep all pictures in my computer?
     
  43. 5263

    5263 G.O.A.T.

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    Important to note about Brian's data.

    It docs the speed coming during a particular joint action, not power generated by that joint. This means that even though the elbow extension phase adds the greatest speed, that speed IMO comes largely from the leg drive up into the launch position and back extension that starts the delayed elbow extension in the best servers like Roddick. Most rec players don't ever hit launch position.
     
  44. toly

    toly Hall of Fame

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    Elbow extension

    If forearm (by using elbow extension) rotates in plane parallel to the racquet string plane, it can produce brushing motion only, which mostly can create just spin. You stated it already many times.
     
  45. spacediver

    spacediver Hall of Fame

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    Absolutely.
     
  46. spacediver

    spacediver Hall of Fame

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    Yes, which is why you pronate to ensure that racquet string plane is perpendicular to the axis of rotation of elbow joint. I've gone over this with you before, Toly.
     
  47. spacediver

    spacediver Hall of Fame

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    www.tinypic.com is the one I use.
     
  48. toly

    toly Hall of Fame

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    What is elbow extension doing on these pictures?[​IMG]]
     
  49. spacediver

    spacediver Hall of Fame

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    hard to tell because of the angle. Looks like the elbow is almost fully extended right from the start, but I can't tell for sure.

    This is an image that shows the elbow extension more clearly. The pronation isn't clearly drawn, but you can imagine that if the player is pronating, then elbow extension will contribute to racquet head speed even though it's not brushing the ball.

    [​IMG]
     
    Last edited: Oct 3, 2010
  50. 5263

    5263 G.O.A.T.

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    who is this server?
    first or second serve?
     

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