one more episode of : yeah right weight transfer gives you power...

Discussion in 'Tennis Tips/Instruction' started by salsainglesa, Aug 4, 2011.

  1. Playnice

    Playnice Guest

    This is what is referred to as "feel".
     
  2. 5263

    5263 G.O.A.T.

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    Not sure elastic collisions are a more relevant way to consider anyway, as they seem to deal with collisions of objects moving with speed and momentum like billiards, but no driving force like a person or motor, to produce accel and/or force thru the contact. (or do they account for that)
    If this is true, does it take away the basis of Toly and MNplayer's chart too?
     
  3. 5263

    5263 G.O.A.T.

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    How do we know this?

    Might there not be other ways to effect mass total without a tight grip?
    Are there different types of tight grips? (meaning to hold firmer in different dimensions)
     
  4. bhupaes

    bhupaes Professional

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    I would say toly's analysis applies to collisions at the center of mass. For a racquet, the center of mass is somewhere near the balance point on the shaft, and a heavy ball hitting the sweet spot would tend to torque the racquet. The hand/body will need to play a stabilizing role by providing a counter torque. The faster/heavier the incoming ball, the more the stabilization will need to be.

    Also, when the pace of the incoming ball is really high... and one is just blocking it back... by toly's analysis, all one needs to do is to get the racquet behind the ball and hold it with two fingers... which is contrary to my experience.

    Edit: Ah, I just digested what toly said... so I will retract the second paragraph above. I thought he said that hand involvement was not necessary, sorry! One should be able to block back even fast serves without getting the body into it.
     
    Last edited: Aug 11, 2011
  5. Fugazi

    Fugazi Professional

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    To the contrary, it's usually important to have a firm grip when hitting a drop volley... Unless the passing shot is very weak.
     
  6. toly

    toly Hall of Fame

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    Inelastic Collision

    I’m very sorry for delay.

    My calculation based on conservation of momentum and total energy of inelastic collision.

    There are two objects:
    First object-Mass M1, speed V1 before collision, and U1 after collision.
    Second object-Mass M2, speed V2 before collision, and U2 after collision.

    We know velocities before collision V1 and V2.

    Unknown velocities after collision can be found from next formulas:

    U1=((M1-R*M2)V1+M2(1+R)V2)/(M1+M2) (1)

    U2=((M1+R)V1+(M2-R*M1)V2)/(M1+M2) (2)

    Where R=(U2-U1)/(V1-V2) – is coefficient of restitution.

    In fact, we are interested in the relative velocity between the first and second object. Then we can suppose V2=0.

    From equations (1) and (2) we have

    U1=(M1-R*M2)V1/(M1+M2) (3)

    U2=(M1(1+R)V1/(M1+M2) (4)

    If M1= ∞, then from (4)

    U2(∞)=(1+R)V1

    Thus coefficient of mass efficiency would be

    U2/ U2(∞)=M1/(M1+M2)

    That's it. It's very simple.
     
    Last edited: Aug 11, 2011
  7. papa

    papa Hall of Fame

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    I was referring to just having a dead ball type shot with the ball just limps over the net and dies.
     
  8. MNPlayer

    MNPlayer Semi-Pro

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    That's a very good point. A purely elastic collision in which the objects are free from outside forces is a very oversimplified model. We aren't throwing the racket at the ball, and there is rotational inertia in the ball, torque on the racket, etc etc.

    I agree, the stabilisation effect of the hand, arm, body seems very important. I suppose one could block hard serves with a loose grip (or by throwing the racket at the ball, in principle), but one would need to be incredibly precise. Perhaps this is where the additional mass of the hand, arm, body comes into play - more of a control feature than power. Of course, more control allows one to use more power safely...
     
  9. MNPlayer

    MNPlayer Semi-Pro

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    As I've already said, it is a highly simplified model. However, conservation of momentum and kinetic energy will be relevant to any model of how this works. F=ma is important but also has limited application, as it applies only to constant acceleration. A more accurate model, I guess, would integrate (in multiple dimensions) the velocity of the racket over the duration of the collision to measure the total force applied to the ball. From this, we could presumably decompose the force vector into a spin-producing component and a translational component.

    The math is not particularly hard, it's the parameters that are tricky. What is the speed of the racket at contact, and how long is the ball collision? How efficient is the collision (what's the coefficient of elasticity)? How much force is the hand applying to the racket at contact, at what vector? What role do the strings play? They have varying coefficients of friction and elasticity of course. Given all that, we could maybe figure this out :)
     
  10. JackB1

    JackB1 G.O.A.T.

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    To the OP....if u don't want to shift your weight into the ball then don't! But why wouldn't you? If you were going to swing an axe into a tree, you would use your core to twist and uncoil, but you would ALSO lean into the tree as you strike it with the axe, wouldn't you? Why not use BOTH rotational forces AND linear forces?
     
  11. mightyrick

    mightyrick Hall of Fame

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    Yes, exactly. We aren't throwing a racquet at the ball. Oversimplified model is generous. It is bogus. It is a misuse of a couple of 200-level physics formulas for balls bouncing off each other (collsions with elasticity formulae).

    You cannot use these formulas (by themselves) to determine the importance of mass as each point in a mechnical or biomechanical kinetic chain. Which is where the discussion was supposed to have started.

    The entire premise excludes the actions that occurred which created V1 and V2.

    Exactly. All of the other factors are ignored. Again, it is the wrong model. The best model for a tennis forehand, given the biomechanics we know are involved in the motion, is a model for a machine of connected levers and cogs. The simplest model would be a catapult.

    [​IMG]
     
  12. toly

    toly Hall of Fame

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    Yes, I agree with this point in general, but not in reality (explanation below).

    About body mass stabilizing role I disagree.
    The most powerful shot in tennis is serve, more than 155 mph. According to the most professional coaches we should keep our hand/wrist very loose (I’m not very sure about that). Thus, maximum “human racquet” mass would be: mass of the racquet, plus mass of the hand, and maybe plus mass of the forearm. This small combined mass is enough to produce 155 mph.
    Also, it looks like ball torque to the racquet is not so important in this case.
    IMO, simple two objects collision model can be used, at least, to get the main quality ideas.
     
    Last edited: Aug 12, 2011
  13. toly

    toly Hall of Fame

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    How this machine works in horizontal plain? Can you explain that?
     
  14. papa

    papa Hall of Fame

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    Yes, I would agree. However, in the discussion that is going on, the grip seems to be the "link" and I don't think you would argue anything different. BTW. As the "link"/"feel" changes, the ball/rebound changes accordingly.

    So, although formulas are nifty, isn't all this stuff dependent on something we really can't measure or quantify? Are we assuming in these matters/calculations that there is no movement at this point (grip)?
     
  15. papa

    papa Hall of Fame

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    Well, the only way I know it is through common sense and playing many years. The tighter/firmer one holds the racquet the further the ball will rebound. For instance, you can hit the same ball to me at net and depending on my grip, I can either just drop the ball over net or volley it to the baseline or whatever (using the same stroke mechanics). My grip would be a significant factor in how much of the incoming shot I'm going to absorb.

    "Are there different types of firm grips ?" Maybe but not to me but then again I might do things that I'm not aware of - probably do. There are many little tricks we all use in grasping the racquet such as the placement of the fingers but I don't think you were referring to that. So, my answer would be no. A "firm" grip is a firm grip, a "loose" grip would be a loose grip and so on. I don't have several types of firm/medium/loose grips.
     
  16. Limpinhitter

    Limpinhitter Legend

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    How does the mass added by upper body rotation factor in to your analysis? I am sure that without upper body rotation, no one can hit a 110 mph serve, much less 155 mph. Further, the swingpath of most groundstrokes involve a combination of both arm swing from the shoulder and UBR. Again, without UBR in groundstrokes, power is significantly lost. It seems to me that UBR does not just add racquet head speed, it adds mass to the racquet and arm.
     
    Last edited: Aug 12, 2011
  17. mightyrick

    mightyrick Hall of Fame

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    Sure, so now you mount the the catapult to the wall. How does the machine change?

    It still has to have a significant counterweight to function. However, the major difference is that gravity cannot be factored in as the force that moves the counterweight. Gravitational force has to be replaced with something else that pushes in a horizontal plane.

    BTW, this is exactly how giant man/horsepowered pumps used to work. You would have some very strong individuals (or even horses/cows) pushing a very large wheel/cog/flywheel in a circle.

    [​IMG]
     
  18. pushing_wins

    pushing_wins Hall of Fame

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    any conclusions for the lay person?
     
  19. Limpinhitter

    Limpinhitter Legend

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    Lay person? "SECURITY!"
     
  20. bhupaes

    bhupaes Professional

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    Let us discount the contribution to the racquet head speed from sources other than those in the arm (upper arm, forearm, hand) for the purposes of argument. Just imagine the arm dangling in space detached from the body (grotesque, I know) trying to create the internal torque necessary to impart the necessary racquet head speed. Clearly, this is impossible.

    IMO, when one says "body behind the ball", what is actually meant is that the body is providing the reactive force the arm/hand system needs to generate a force that can be applied to the incoming ball. This is what the hand-body connection means to me. This connection can be relatively stronger or weaker depending on your mechanics, but it's there in some form at all times. If there is something deeper than this, I would love to be enlightened.

    Once the hand/racquet system has been accelerated, your analysis kicks in. What I was pointing out was that even then, because of the shape of the racquet and the constituents of the arm, the analysis is not simple. I think mightyrick's model is the right one. If you have played with a very light, headlight racquet against very heavy serves or shots, you will know what I mean. At contact time, whether you like it or not, your grip will tighten as you are forced to provide the counter torque. If you don't, the ball will not go where you want it to go...
     
  21. Rogael Naderer

    Rogael Naderer Semi-Pro

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    Yeah, to gain lots of power...

    Go and practice.
     
  22. peoplespeace

    peoplespeace Semi-Pro

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    so true, even if it is just a few inches, the majority of the energy comes from that not the rotation. One of the reanons for thescissors kick is to reduce the rotation and prolong the linear momentum. The ball is only in contact with the strings for few miliseconds, so the time it takes to move a few inches forward is plenty, if well timed, for the ball to be mostly inpacted by linear momentum more that rotational momentum even if the naked eye sees a person who is mainly rotating as in the case of fed inside out forehand.

    And thank u to mightyrick for all the great examples, loved them! :)
     
  23. nabrug

    nabrug Rookie

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    Can you only push off from the side of a swimming pool when the side first moves towards you? (For maximal power you want that to happen but in this case you only have to make the ball trajectory.)
    No you can also push off that side when it is maintaining it's position.
    You can even push off if it is moving backwards. You can find a lot of examples that Roger is performing the same stroke while really moving backwards.
     
  24. mordecai

    mordecai Rookie

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    Angular momentum from powerful hip rotation is the engine for modern serves and groundstrokes. Leg drive not only facilitates hip rotation, but adds topspin. Having time to add either linear weight transfer or a deeper coil in the takeback is the difference between a neutral and attacking ball.
     
  25. Fugazi

    Fugazi Professional

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    You... guys... lost... me... feeling dizzy... can't breathe...
     
  26. toly

    toly Hall of Fame

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    To produce the most powerful strokes, we should grip the racquet handle tight enough to add to the racquet - masses of the hand and forearm. At the same time, in case of FH, we have to make available the counterclockwise hand rotation around the wrist joint and avoid, during impact, involuntary clockwise rotation. IMO the semiwestern grip is the best for these conflicting tasks.:)
     
    Last edited: Aug 13, 2011
  27. aaron_h27

    aaron_h27 Rookie

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    His back foot is off the ground which means he's leaning into his shot which means his body weight is going into the shot no?

    Backfoot off the ground = Weight Transfer IMO
     
  28. mightyrick

    mightyrick Hall of Fame

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    No. In frame 1, his left shoulder is in front of his body. In frame 3, his left shoulder is in behind his body. This means that he has rotated his entire upper body mass as a counterweight to his arm -- pulling his arm along. Just like a catapult.

    The important weight transfer is rotational.
     
    Last edited: Aug 13, 2011
  29. Limpinhitter

    Limpinhitter Legend

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    I'll try again! In post #162, you stated:

    In post #166, I asked:

     
    Last edited: Aug 13, 2011
  30. toly

    toly Hall of Fame

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    I’m sorry for delay. I absolutely agree with you about body, arm, wrist etc rotations. I’m big enthusiast of the rotation motions in tennis strokes, because they can produce much bigger racket speed than straight linear motions.
    But, I cannot understand how UBR can add mass to the racquet and arm. For instance, can UBR add the mass to the left arm too?
     
  31. toly

    toly Hall of Fame

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    Our joints are very good with rotation, but very bad with straight linear movement. Additional forward momentum of the player creates a little speed, and just can distract him/her from creating proper angular motion. If you can do both, you are lucky. But, Federer practically cannot or doesn’t want to!?
    The main idea is: Rotate relevant body parts as fast as possible, do not muscle the stroke, and let the racquet fly without restraint.
     
    Last edited: Aug 13, 2011
  32. Limpinhitter

    Limpinhitter Legend

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    I'm a big advocate of UBR in both serve and groundstrokes. I'm not certain that it does add mass to the racquet and arm. But, I think it does. Imagine that there is no shoulder joint, only an arm extending out in a fixed position. Wouldn't the mass of the upper body add some mass to the racquet and arm when rotated? If so, then wouldn't a semi-fixed shoulder joint (ie: limited arm swing), resulting in a balanced combination of UBR and arm swing result in more mass added to the racquet and arm than an arm swing with little or no UBR?

    I have long observed (playing tennis since 1969), in my own shotmaking that I can generate more power with an equal combination of UBR and limited arm swing from the shoulder, even if it means sacrificing a bit of racquet head speed, than I can from using mostly arm swing and more racquet head speed. On serve, I call lack of sufficient UBR Jimmy Connors syndrome. Or, perhaps UBR is merely adding racquet head speed that I can't percieve when swinging, and it has nothing to do with mass. But, somehow, I think it does have something to do with mass.

    Put another way, although you and I probably agree that rotational momentum is more effective than linear momentum when applied to groundstrokes. But, wouldn't linear momentum in which the entire body moves in the direction of the swing in the contact zone add mass to the racquet and arm? Or is it only that it adds racquet head speed?
     
  33. mightyrick

    mightyrick Hall of Fame

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    In physics, you can't add mass to any object. The important factor here is force. Force is a product of mass an acceleration. At each point in a kinetic chain, energy(force) is converted to acceleration and/or pressure.

    A rotation of 120 pound (45kg) upper body at 10mph would create about 900 Newtons of force. When this 900 Newtons is applied to the shoulder joint, it is converted into acceleration and pressure on the arm. Since the arm only weighs 9 pounds... the remaining energy will be converted into acceleration (and some lost in the transfer). And that isn't counting the additional acceleration/pressure added by the shoulder itself -- which is a very strong muscle.

    But let's be overly simple and work backwards from the point of contact. If we know that racquet head speed for a professional forehand is 100mph, and we know that a racquet weighs about 12 ounces, we can calculate the force at impact.

    F = m * a
    F = .39kg * (44.7m/s ^ 2)
    F = .39 * 1998
    F = 779 Newtons

    779 Newtons (the ending force at contact) and the 900 Newtons (force created at the shoulder) are pretty close. It makes sense that there would be some lost net energy in the transfer along the kinetic chain.

    So in the example model I just asserted that 900 newtons are initially created by the body rotation and transferred into the shoulder. Along the rest of the chain (shoulder, elbow, wrist, strings, ball)... there is approximately a 12 percent loss of energy.

    I am greatly oversimplifying a highly complex chain of pulling, pressure, force, and acceleration... but even still... it isn't that far off.
     
  34. toly

    toly Hall of Fame

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    Limpinhitter
    I'm a big advocate of UBR in both serve and groundstrokes.
    I’m not very big advocate of UBR due to it is very slow motion. It can produce big force but very low angular speed. I explained this issue in thread http://tt.tennis-warehouse.com/showthread.php?t=361610 post#1.
    I'm not certain that it does add mass to the racquet and arm. But, I think it does. Imagine that there is no shoulder joint, only an arm extending out in a fixed position. Wouldn't the mass of the upper body add some mass to the racquet and arm when rotated? If so, then wouldn't a semi-fixed shoulder joint (ie: limited arm swing), resulting in a balanced combination of UBR and arm swing result in more mass added to the racquet and arm than an arm swing with little or no UBR?
    You are right. Our body can add mass to the “human racquet” assuming that arm is in fixed position. If we stand on the ground we can even add the mass of our planet. But, arm and racquet themselves can hit the ball with mass efficiency approximately equal to 99%. Thus, there is a little sense to involve body in additional mass formation.
    I have long observed (playing tennis since 1969), in my own shotmaking that I can generate more power with an equal combination of UBR and limited arm swing from the shoulder, even if it means sacrificing a bit of racquet head speed, than I can from using mostly arm swing and more racquet head speed. On serve, I call lack of sufficient UBR Jimmy Connors syndrome. Or, perhaps UBR is merely adding racquet head speed that I can't percieve when swinging, and it has nothing to do with mass. But, somehow, I think it does have something to do with mass.
    Sure, UBR can create liner racquet speed alone. It can be calculated according to the next formula
    Vrac = R*Ѡ
    Where R is radius of rotation of impact point on the racket string bed and Ѡ is UBR angular speed.
    But, unfortunately, Vrac usually is very low. I prefer using much faster motions of the arm and it’s parts and of course I'm also using UBR.
    Put another way, although you and I probably agree that rotational momentum is more effective than linear momentum when applied to groundstrokes. But, wouldn't linear momentum in which the entire body moves in the direction of the swing in the contact zone add mass to the racquet and arm? Or is it only that it adds racquet head speed?
    Straight line body motion is very slow. If I put the racquet on my belly and jump forward into ball, I believe I make roughly 10 mph “belly racquet speed”, but I waste a lot of energy, which I better use for rotational motions. They can create 100 mph. Forward jump also would be very bad for recovery if I’m not going to run to the net.
     
    Last edited: Aug 15, 2011
  35. sureshs

    sureshs Bionic Poster

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    Why is the accelration the square of the velocity?
     
  36. toly

    toly Hall of Fame

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    Do you have only one question?:)
     
  37. mightyrick

    mightyrick Hall of Fame

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    I screwed up and keyed the square (^2) straight into my calculator. I was moving way too fast while typing the post so my session wouldn't time out.

    But ultimately, I was only trying to show (poorly) that the initial rotation of the mass of the upper body puts a large force onto the shoulder. Given the mass of the arm, the force placed on it will result in acceleration. Since the arm has far less mass than the upper body, the acceleration will be large. The arm is significantly lighter than the racquet, so when the arm transfers its energy to the racquet, the acceleration is even larger.

    But my whole point was that the initial force comes from the body rotation. Without the force of the body rotating... only the shoulder could provide the energy. And as others have said... by using only the shoulder, you could never swing a racquet at 100mph. Why? Because the shoulder has far less mass and cannot (by itself) create enough force.
     
  38. vincent_tennis

    vincent_tennis Professional

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    lololoololol

    he means ms^-2
     
  39. vincent_tennis

    vincent_tennis Professional

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    Regrading your equation, how did you determine the acceleration only with the final velocity? -_- the acceleration would depend on the players are length, swing path etc etc~

    (just curious)
     
  40. mightyrick

    mightyrick Hall of Fame

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    It was just an extremely simplified frame of reference. The in the first instant of the swing, the velocity is zero (or nearly zero). At the second frame of reference, the velocity is the speed of the racquet head. Again, only used for an example of calculating the force. I've already said before that nobody can accurately calculate any of this without doing serious testing at every point in the kinetic chain.

    But the only point is that force is the relevant factor. And that the initial force is placed from the upper body to the shoulder. After that, it is transferred to the arm, wrist, and racquet.
     
  41. toly

    toly Hall of Fame

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    Serious testing and research have been done already . For example, see please http://www.racquetresearch.com :)
     
    Last edited: Aug 15, 2011
  42. mightyrick

    mightyrick Hall of Fame

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    Honestly, I think a better site is here (although this is communicating between bat and ball). http://www.kettering.edu/physics/drussell/bats-new/impulse.htm. I like this site because it accounts for the speed of the incoming ball -- which generates a larger impulse.

    Although neither site deals with rotational force being responsible for the swing force.

    Again, I refer to baseball sites because I think they address this far better. Mainly because "rotational hitting" is an actual method of swinging in baseball.

    http://www.backbackback.com/newtonwilliams3.htm
    http://www.batspeed.com/research10.html
     
  43. 5263

    5263 G.O.A.T.

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    I think this is very good, and is key to the right answers.
     
  44. zapvor

    zapvor Legend

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    wow reading a few pages in here...we have a real actual legit thread on TT!

    my question is...does solving this problem will actually...improve our game:D
     
  45. PandaKing

    PandaKing New User

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    In the end, no. Only because no matter what, the squirrel will protect his nuts from the evil raccoons.
     
  46. mightyrick

    mightyrick Hall of Fame

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    Good point. Dedicating time to this thread has not increased my ability to consistently putaway short floaters.

    It would be nice if knowledge could just translate into ability in sports. :D
     
  47. 5263

    5263 G.O.A.T.

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    Not true for all.
    It was threads like this which introduced me to Oscar's methods, even though before, my teaching and play were very classical. Discussions like this one led me to investigate some of the modern techniques due to some of the excellent points that were raised.
     
  48. 5263

    5263 G.O.A.T.

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    Why would this not help you with floaters?

    What happens when you attack mid court balls? doubles or singles?
    Most miss due to off balance or stepping in too strongly.
    This thread addresses that well.
    Some miss due to poor target management.
    There are other reasons of course.
    How do you miss?
     
  49. Mahboob Khan

    Mahboob Khan Hall of Fame

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    Many factors contribute in hitting the ball with power:

    -- the knee bend (to push against the ground)
    -- the upper body turn (coiling and uncoiling)
    -- the weight transfer as you hit
    -- the racket head speed (arm speed).
    -- Using the speed of the ball i.e. hitting the ball on the rise.

    Mahboob Khan
     
  50. 5263

    5263 G.O.A.T.

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    from your 3rd reference-

    Dedicated coaches and players spend countless hours in batting practice to perfect their batting performance. But regardless of how dedicated they may be or how hard they work, a hitter can never reach his or her potential while practicing with inefficient swing mechanics. For decades, batting coaches have taught batting mechanics based on a linear weight-shift and extension theory.
    Many batting coaches have based their teaching on the belief that bat speed is developed from a forward weight shift and an "A to B" extension of the hands (or "knob first"). They believe that, as outlined above by Mr. Adair, as the hands slow, there is a transfer of energy (or momentum) that accelerates the bat-head .

    However, lab tests prove that there is no whip, flailing or pendulum effect of the bat-head generated from a forward weight shift and straight extension of the hands.
    and
    NOTE: Linear mechanics gives a batter the illusion of power. --- A great hitter experiences the centrifugal pull of a highly accelerated bat head.

    IMO the batting comments hold even more true in tennis, where the rotation aids in control and court coverage as well.
    nice links, thanks
     
    Last edited: Aug 16, 2011

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