"Accelerate evenly through the ball"

Discussion in 'Tennis Tips/Instruction' started by Bergboy123, Apr 27, 2012.

  1. bhupaes

    bhupaes Professional

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    First and foremost, all this physics stuff should be purged out of one's mind while playing - let's be clear about that! :)

    That said, I make sense out of all of this as follows. If you recall how I defined extension (in the extension discussion a few days ago), I said extension reaches a maximum just before impact. At that point, be it a forehand or serve, the hand can go no further, and reaches a maximal point where it slows down a lot. This sudden slowing down of the hand releases the wrist, and the racquet head accelerates to impact. At some point after impact, the wrist can release no further and starts dragging the hand/forearm, which causes the hand/forearm to pick up speed after impact into the follow through.

    Well, what does this imply for coaches... that's a good question! I would say a really relaxed wrist, and a good runway into extension by laying the wrist back so there's enough room to accelerate the racquet head. I know I'm not being precise here, but you get the idea.
     
    #51
  2. chico9166

    chico9166 Guest

    If i understand you correctly, then I'd hate to see you use a hammer, or axe, or any other tool. Does the handle move at the same speed as the end point?
     
    #52
  3. 5263

    5263 G.O.A.T.

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    This is exactly what we are trying to avoid from traditional tennis.

    Not sure how you got that from me saying the hand is what we directly control, but the control of the hand and the technique is what brings the racket face around with acceleration beyond the hand.
    We don't have direct control of the racket face, but control thru technique and
    via the handle thru the shaft.
     
    #53
  4. TennisCJC

    TennisCJC Legend

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    Man, this has gone off the deep end.

    If you want to hit a controlled rally shot, accelerate thru the contact zone with a smooth stroke. If you want to hit a harder shot, accelerate thru the the contact zone at a faster pace. Neither is slow and neither is out of control. The concept of accelerating thru eliminates deceleration which is the kiss of death on groundstrokes and serves.

    I think the charts and graphs probably say the same thing but they hurt my head. Swing thru the damn ball and finish strong.
     
    #54
  5. 5263

    5263 G.O.A.T.

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    Pretty good way to say it.
     
    #55
  6. sureshs

    sureshs Bionic Poster

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    Double pendulum seems to be the wrong model. Look at the clip - the bar representing the lower arm almost stops while the racket continues moving up into the ball. It happens because the racket has energy from its swing on the other side.

    Now, try to hit a top spin forehand by decelerating and then stopping the lower arm when it is about vertical and tell me whether the racket moved up by itself and hit the ball!

    That is why you get the wrong plot that hand velocity is 0 when racket velocity is maximum.

    I have objected to the pendulum analysis when it was put forward in the polarization threads.
     
    #56
  7. 5263

    5263 G.O.A.T.

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    I was noticing this too.
    You can do it and many beginners start something like this with very poor results, lol.
     
    #57
  8. sureshs

    sureshs Bionic Poster

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    In the light racket clip, the racket does not move up much. I was kind of OK with that, except I felt what purpose does it serve. Then the heavy racket clip showed the racket moving up much more, and again I was like, OK, but who hits the ball like that?

    I can imagine that this model MIGHT describe a golf put, but not a tennis shot. Even feeding the ball requires more effort than that.

    The hand and the racket have to move together - one cannot stop and hope that inertia will swing the racket up into the ball just by keeping a loose wrist.
     
    #58
  9. 5263

    5263 G.O.A.T.

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    THis is also why I did not feel I could depend on the data, especially
    in reference to the hand slowing in the graph.
    I think with a true modern swing, you will see the speed stay up there better
    with the hands and the racket, also
    for a slightly longer time than Cross' graph shows.
     
    #59
  10. sureshs

    sureshs Bionic Poster

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    I think it was a case of picking the only studied mathematical problem which even remotely resembles the real case. You see this in textbooks, where the purpose is to give the student a very broad idea of the actual thing to help them visualize. But in this case, the analysis does not seem to be useful to anyone.
     
    #60
  11. julian

    julian Hall of Fame

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    Clarification

    Cross uses the term "angular velocities of the forearm and the racket"
    in the caption of Fig 4 of
    http://www.physics.usyd.edu.au/~cros...ennisDPend.pdf
    Both of them are obviously scalars,NOT VECTORS
    These terms are used,I believe.first time at the caption of Fig 1a of the same reference
    It comes without saying that the ratio of rotational energy of the racket/rotational energy of the arm
    has good physical meaning
     
    Last edited: Apr 30, 2012
    #61
  12. bhupaes

    bhupaes Professional

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    The hand/forearm slowing down (not stopping!) before contact is very obvious in the serve.

    For the forehand, what seems to be happening is that some time before contact there is a similar slowing down due to maximal extension. Also, the hand starts pulling the racquet in, which causes the forward component of the hand velocity to decrease, thus releasing the wrist and accelerating the racquet head - which is what matters. IMO this is what Rod's charts and graphs are showing, when you look at it qualitatively. The double pendulum theory and its associated equations would be useful for quantitative analysis, I suppose.
     
    #62
  13. 5263

    5263 G.O.A.T.

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    Now this could be an interesting perspective the subject.

    Sure would like to get a look at their data with this!
     
    #63
  14. 5263

    5263 G.O.A.T.

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    Would be cool to see this data with the 1st pendulum being pulled back at the right time or better just continuing in a tighter radius which pulling right would give. By the way, this change of direction alone is accel in science terminology.
    Right?
     
    #64
  15. bhupaes

    bhupaes Professional

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    Absolutely - changing the direction of a moving object requires the use of force, which by definition results in acceleration.
     
    #65
  16. bhupaes

    bhupaes Professional

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    Toly, I stole your beautiful picture! Sorry!

    [​IMG]

    To make my point, you can see how the extended forearm has moved very little between frames 1 and 8, yet the wrist has come around completely.
     
    #66
  17. sureshs

    sureshs Bionic Poster

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    If you are saying this is like a double pendulum, it cannot be. A pendulum uses gravity, and here the motion is mostly in the horizontal plane, not vertical. The action has to be forced by the forearm and the wrist, and is not passive.
     
    #67
  18. 5263

    5263 G.O.A.T.

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    I think it is just a bad camera angle mostly and is not supported by anything I've seen from better angles or from above. Evidence of the across aspect is in how the ball has moved from the throat area all the way
    out to the outer part of the upper hoop in just these few fast frames.
     
    #68
  19. user92626

    user92626 Legend

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    I'm with 5263. That has to be a bad cam angle or Fed was doing some sort of ultra speed slappy inside out fh to take away time from opponent. It cannot be representative of a regular, consistent and sound FH, ie hitting with a nonpassive forearm and wrist as suggested.
     
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  20. SystemicAnomaly

    SystemicAnomaly G.O.A.T.

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    Very interesting. I imagine that it uses imbedded accelerometers. It would be cool if they could add some other iPod features a well (down load mp3's to your racquet?).
     
    #70
  21. sureshs

    sureshs Bionic Poster

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    Yes, he is improvising from outside the alley to angle the ball away to such an extent that the opponent cannot reach it, otherwise he will be toast in the open court.
     
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  22. user92626

    user92626 Legend

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    Good point/observation. No decent pro, let alone Fed, plays a usual shot and a usual FH from that position. That must be the last shot or he's screwed.
     
    #72
  23. sureshs

    sureshs Bionic Poster

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    Do you think he is punishing a loose second serve or is this a groundie?
     
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  24. bhupaes

    bhupaes Professional

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    This is an extreme case, I agree. Fed's normal TS FH would show more across movement (you can see some pulling of the racquet inwards here also, as you have pointed out). But this illustrates the point I was trying to make very well - the slowing down of the arm, the across movement, and the accompanying acceleration of the racquet.
     
    #74
  25. SystemicAnomaly

    SystemicAnomaly G.O.A.T.

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    I would think that an "intent to accelerate" would refer to increasing the speed of the stringbed/racket head rather than the hand. It's the collision point where we want to transfer energy/momentum to the ball. I would think that it could refer either to the serve or the FH. However despite our best efforts, the racket head loses speed at contact. By attempting to "accelerate thru contact", we ensure that we are still accelerating just prior to contact. Note: if we truncate our follow-thru, there is a good chance that we are slowing down just prior to contact.

    As for the data and the graphs:

    The data for the TWU graph that I show in post #41 is the very same data from the very same source that is presented in Fig 4 of your first links. The only difference is that y-axis (the angular velocities) is expressed in different units and the time axis is truncated at 0.3 seconds for Fig 4. It is my belief that the indication of the "impact time" is an overlay -- something that was tacked on to the graphs.

    Figure 4 is from a longer document from 2010 while Rod Cross's TWU article was published in June of 2011. It is my belief that the impact time indication is a goof that was overlooked in the longer document and subsequently corrected in the later document.
    .
     
    Last edited: Apr 30, 2012
    #75
  26. SystemicAnomaly

    SystemicAnomaly G.O.A.T.

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    But then gravity is just really just a force. In this case the force is provided by a different means. The double pendulum idea might be something of a first order approximation of what is happening. Would a trebuchet provide any further insight into what is happening?
     
    #76
  27. sureshs

    sureshs Bionic Poster

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    When the pendulum is moving up, gravity still acts downwards, pulling it back. It is not at all like the force supplied by the hand.
     
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  28. toly

    toly Hall of Fame

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    I hope that in the end you realize that in case of Federer flat FH passive/active hand action can contribute more than 50% of the power. :)
     
    Last edited: Apr 30, 2012
    #78
  29. user92626

    user92626 Legend

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    Given how deep inside the court, probably to finish a groundstroke exchange.
     
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  30. bhupaes

    bhupaes Professional

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    Let's say the exact percentages of passive and active contribution are still under consideration - or should I say negotiation? :) There's no way to tell from pictures, as far as I know!
     
    #80
  31. toly

    toly Hall of Fame

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    #81
  32. 5263

    5263 G.O.A.T.

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    Nice vid, but made it even more clear how strongly he is working across that ball. It probably had nearly pure side spin on it.
     
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  33. toly

    toly Hall of Fame

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    It is very well known fact; momentum has the special property that, in a closed system, it is always conserved. Kinetic energy, on the other hand, is not conserved in collisions if they are inelastic.:shock:
     
    #83
  34. toly

    toly Hall of Fame

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    From physics and math point of view, there is practically no tangential component of the racquet point of contact velocity (vector) during impact. Thus, there is no spin at all. Do not argue with the lows of physic please. It doesn’t make any sense. I’ll try to provide picture which (maybe) support my explanation. :(
     
    Last edited: Apr 30, 2012
    #84
  35. SystemicAnomaly

    SystemicAnomaly G.O.A.T.

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    Already made that point a while back.

    My comment was regarding the way that some other posters were using the term, acceleration. I don't believe that they were referring to a change in direction in their use.
    .
     
    Last edited: Apr 30, 2012
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  36. SystemicAnomaly

    SystemicAnomaly G.O.A.T.

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    Have you discussed this with Rod Cross? I have several email addresses for him if you wish to try to contact him.
     
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  37. 5263

    5263 G.O.A.T.

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    I try not argue with Physics, but when it is different than the event, it makes me question.

    Are you suggesting he hit a knuckle ball here?

    I'd be willing to bet a lot the he got some good spin here and with a good component of side to it.
    The vid showed the racket face moving from outside the ball to inside pretty quickly.
    Sorry, but I think you are trying to prove a faulty premise on how the wrist is used, and it is
    clouding your objectivity.
     
    #87
  38. 5263

    5263 G.O.A.T.

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    But you can be sure I was referring to change of direction, as that is the basis
    of what we are saying about the modern Fh in MTM.
     
    #88
  39. SystemicAnomaly

    SystemicAnomaly G.O.A.T.

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    ^ I believe that the OP and others were not using the term to indicate a change in direction.

    .
     
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  40. 5263

    5263 G.O.A.T.

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    that is very reasonable, lol.
     
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  41. SystemicAnomaly

    SystemicAnomaly G.O.A.T.

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    Way cool! I contacted Rod Cross about the apparent discrepancy between his 2 graphs that show the angular velocities of the racquet and arm. It took him less than 45 mins to get back to me with a detailed explanation. Here it is:

    Both graphs are actually the same graph, but I have labelled the impact time in different spots as you correctly point out. The main point that I wanted to show was that the forearm speed drops quite a lot before the player hits the ball. That is probably quite surprising but it makes sense since rotational energy is taken out of the arm and given to the racquet just before the racquet strikes the ball. It would also make sense if the ball is struck when the velocity of the racquet head is at a maximum. Whether that actually happens is a bit uncertain, but it probably does happen. It is uncertain for two reasons:

    1. The velocity of the racquet head consists of two parts. One is the part due to pure rotation of the racquet. That is the part shown in both graphs. However, the racquet head is also moving forward because the whole arm is moving forward. In fact, it is possible to strike a ball without rotating the racquet at all, simply by pushing the racquet head forward, as players used to do in the old days with wood racquets. So, the exact time at which the racquet head has maximum speed could be before or after at the same time that the rotation speed (ie angular velocity) is a maximum.

    2. In Fig. 4 in the AJP paper, I calculated the angular velocity a bit beyond the impact time, but I did not consider the effect of the impact itself. That is, I let the racquet swing as a double pendulum without actually hitting a ball. If the racquet did hit a ball then the angular velocity would drop suddenly. The sudden drop is not shown in Fig 4 because I was only interested in calculating the speed of the racquet resulting from the forces exerted by the arm. I did not include the effect on rotation speed due to the force of the ball on the racquet.

    The impact time labelled in Fig 4 is the point where the racquet is lined up correctly to strike the ball as shown in Fig. 3.

    The impact time labelled in the TWU article assumes that the racquet is lined up correctly when the angular velocity is a maximum, in which case the ball will come off the racquet at a different angle. In the TWU article, the angular velocity drops after the impact time, not because the racquet hit the ball but because that is what happens even if the racquet doesn't strike a ball. That is the expected behavior of a double pendulum that oscillates back and forth naturally without hitting anything. If the racquet hit a ball then the angular velocity would drop very rapidly, in 0.005 seconds, to about half its pre-impact value.

    Rod Cross
    .
     
    Last edited: Apr 30, 2012
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  42. 5263

    5263 G.O.A.T.

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    very interesting peek inside there SA!

    Also explains how we can't read too much into this data, as it was not clear there was not an actual impact (although I had suspected that given the graph), even though maybe that info was in there somewhere.

    And thanks Rod, for sharing that with us.
     
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  43. sureshs

    sureshs Bionic Poster

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    So is it true that the forearm slows down before impact? I am not just not able to digest this.
     
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  44. julian

    julian Hall of Fame

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    Reduction of the speed of arm

    It looks like the forarm speed drop depends heavily on values of couples C1 and C2
     
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  45. toly

    toly Hall of Fame

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    You cannot digest this because this is not true.

    Rod Cross just calculated conversion of potential energy into kinetic energy for the simplest double pendulum model. It has absolutely nothing to do with real tennis strokes. There are practically no correlation between real tennis data and Rod Cross double pendulum model of tennis strokes.

    IMO, it would be good idea to forget about this stuff completely.:)
     
    Last edited: May 1, 2012
    #95
  46. jmjmkim

    jmjmkim Semi-Pro

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    Same with a golf swing. That's why a pro swing looks effortless, and so easy. They start the swing with just the natural unwinding of the body coil, and then release the "wrist" just before it reaches the ball, thus producing tremendous amount of leverage and club head acceleration at impact. In tennis, it is this acceleration "thru" the ball that makes that "POP" sound when good players hit.

    This is much, much harder to do on the move, or on the run. That's why without the footwork to get you in place ahead of time and initiating the swing process before the ball comes, you produce weaker and inconsistent shots.
     
    #96
  47. olliess

    olliess Semi-Pro

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    I think the "POP" is just because they can bash the cr*p out of the ball without hitting the frame. ;)
     
    #97
  48. sureshs

    sureshs Bionic Poster

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    I also think so
     
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  49. 5263

    5263 G.O.A.T.

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    And you started out so well here, lol.
    Sure, it's far from perfect, but I think there are some reasonable correlations that can be observed.
    You and Suresh are probably right that it is a good Idea not to be too concerned over it though.:)
     
    #99
  50. julian

    julian Hall of Fame

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    Another analyzer?

    http://www.mytpi.com/mytpi05/tpi3d/category.asp?id=104
     

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