Soderling's Backhand; Video Available?

[uspta990770809]

Dynamics diagrams

Contentiousness gets a little old,but this has been a great thread. It's almost as if we need an outside moderator to put each argument up on a board where we could see all the different arguments simultaneously. It seems as if answers are just slightly missing the questions presented, and yet a lot of information is getting laid out. I'm trying to follow along, but it's tough.

When I was in my first basic Dynamics class at Harvey Mudd with Dr. Alford back in 1967, the first thing you had to learn to do was to draw a force diagram. The trick was to include all the forces acting on the body you were concerned with, in this case, the ball. Those forces would be represented by vectors. We were still using slide rules (really!) in those days and now you probably just hit a couple of keys in the appropriate software and the diagram will appear including size and direction in whatever units you would chose.

The diagram of the ball would have a vector for gravity and perhaps one for wind. It would also have one that represented the force it carried from the momentum its movement gave it before impact. But at the moment of impact, the key outside force would be the force vector of the racket at the impact point. Also critical would be what portion of that force vector gave lift and what portion gave forward velocity. Except for gravity and initial up and down movement of the ball, the primary movement of the ball after impact will be in a direction perpendicular to the face of the racket in all three planes. The most powerful shots will be where the momentum vector of the racket head is the same as the directional vector of the string bed. I'm getting a little carried away, but that is it in simple terms. The ball doesn't know how heavy the hitter is or how big the racket is. It only knows, and for only about 4 milliseconds, that it has an incoming velocity and direction and is being influenced by gravity and the force at the little bit of the string bed that it is touching.

You all know what it feels like to hit a "sweet" shot where the ball just takes off effortlessly and you feel tremendous power with little effort. That happened because you lined up the vector of the racket face and the vector of the momentum of the racket head and they went where you wanted the ball to go (or rather a little above that because of gravity)!

I'm going to let someone else take it from there,
don

[rosooki]

F = Ma

No, I'm saying that there is only one M in the equation. If M is the mass of the player's body then the equation is talking about the acceleration of the body around the court. No other M can be involved, no tennis racket, no ball, no nothing.

F = MA describes, for example, how much F is needed to accelerate your car M at a rate of 5 mph/ph. It says nothing about the effect of the car on anything it might hit.

[crass_lawner]

Crass Lawner, I recommend you talk to someone who understands the equation. You are misunderstanding it. There are only three things, the mass 'effected', the acceleration of that mass, and the force needed to accelerate it.

You keep misunderstanding the mass to be the mass of the body. If it was, the equation would say that to F is the amount of force needed to accelerate the body M by A.

I recommend you read my post. I do understand the equation. In your post I assume you mean the mass 'effected' to be the tennis racket. What I clearly state in my post is that the force needed to accelerate the racket comes from the body (through the hand and the kinetic chain blah blah blah).

I don't have time now so I'll respond to your last post later.

[rosooki]

Mass again

So far there has been the assumption that more mass in a player equals more strength, more backstop, more pop, etc. So we should actually be talking about those things instead of mass. We should be talking about the total quality of the machine instead of just the mass of the machine.

If we are talking only about mass, the only mass that matters is the amount delivered to the ball, not the total amount in the system. We can imagine a robotic tennis arm made of steel with a racket on the end. The arm currently hits the ball just fine at speed x. Doubling the weight of the arm without affecting the speed the robotic arm swings or any other variable will have a linear effect on the ball if the speed didn't slow down do to the heavier weight - the engine will have to be improved to move the heavier mass at the same speed as before.

The arm's mass doesn't matter that much once it has the minimum mass necessary to support the maximum performance of its engine. Increasing the mass after that will only slow it down, since the maximum performance of the engine has been reached. Raising the mass without improving the engine won't be helpful. Conversely, improving the engine without changing the mass will allow a faster swing.

We can imagine a wrecking ball on the end of a crane. The mass of the crane only matters in its effect on the wrecking ball – if any. It is the size and speed of the wrecking ball that actually brings down the building. A given wrecking ball imparts a certain amount of energy per hit on the building, and that's the force that matters. Doubling or tripling the size of the crane won't do anything - if you keep the same size engine in it. No one ever considers whether the crane should be as big as the building to more effectively knock it down. Instead they worry about whether the ball should be bigger or should be swung faster. It is the wrecking ball that matters – if I can swing that very fast with a crane the size of a tonka toy, I certainly will. I leave it to you to decide if swinging the wrecking ball faster or making it more massive will be more efficient in knocking the building down.

Looking at the specs of ball machines, I can’t see any relationship between the weight of the machine and the maximum advertised speed the ball machine serves the balls at. As technology improves, the machines will probably keep getting lighter until they reach the physical limit imposed by the weight of the tennis balls. At which point they would start to move backwards due to recoil.

I’m not suggesting anything is impossible. It may or may not be true that mass matters for a particular player, but it is not a rule. I doubt it is true for me - I'm 6'2" and full of muscles (although I've never smiled and gave anyone a vegemite sandwich), but I have no illusions that doubling my mass would allow me to hit the ball with more power. It would probably make me hit it more weakly. I'm also pretty sure Santoro could dominate me and blow me off the court. If the efficacy of mass depends on the system, it is not a given that more mass is better, and therefore the mass in the body can't be used in any physics calculations - we don't know the relationship of the mass of the body to the effect on the ball well enough to make any sort of equation.

Then there’s always the twilight zone – where Murray’s running shots come from.

[rosooki]

F = MA again

Crass Lawner, I recommend you talk to someone who understands the equation. You are misunderstanding it. There are only three things, the mass 'effected', the acceleration of that mass, and the force needed to accelerate it.

You keep misunderstanding the mass to be the mass of the body. If it was, the equation would say that F is the amount of force needed to accelerate the body M by A.

[crass_lawner]

This post is a response to rosooki's last post:

Actually, Crass_Lawner, the mass involved in both equations is the mass at the center of gravity of the moving object. In this case, that would be the point on the tennis racket where the ball strikes it, not the person's body, which is not part of the equation at all. It is the racket's mass that matters, not the mass of the person. The mass of the person may or may not matter for accelerating the racket up to a given velocity depending on biomechanics, skill, strength, etc. but it is not a given that a fat (massive) guy of strength and skill x is going to hit the ball harder than a thin wiry guy of the same strength and skill. If it is not a given, it is not a rule.

Secondly, the racket may be accelerating through the shot, but there is a specific velocity that the racket hits the ball at at the moment it imparts its energy to it. That is the velocity we are interested in, not the velocity before the hit, or after the hit, or how fast (acceleration) that velocity is changing.

Ceterus Paribus (All other things being equal) - that's the magic sentence in dealing with holding down complexity in any science, velocity matters much more than mass. Yes, mass matters. Why? Because the ball we are trying to impart energy to has a specific set mass and our racket mass has to be set with that in mind. A supersonic toothpick won't be optimal. There will come a point where the mass is so small that increases in velocity would have to be huge to overcome the difficulties created. So given equal rackets, equal strings, equal arms, wrists, technique, strength, wind, margarine spread on the toast eaten for breakfast, etc. changes in velocity will yield greater changes on results than changes in mass.

As for my "flippant comment" my point was that one process you might follow would be to find the maximum swing speed your body is capable of in any given situation, then choose the heaviest stick you can swing at that speed - reliably and consistently. If you are really enthralled by mass, try the process the other way around and see how that works for you. My guess is that it won't.

First of all there's no way that the mass involved in both equations is the mass of the point on the tennis racket where the ball hits it. The mass of the point on the tennis racket where the ball hits it is the mass of a small tennis ball sized circle of your strings. Perhaps you meant relativistic mass of the contact point, but that's a complicated concept that I'm sure you don't understand (as top physicists today disagree as to its definition and existence) and is surely dependent upon body mass. Or perhaps you meant weight, but weight is only dependent on gravity, not movement, so technically that isn't the correct term. The energy contained in the center of gravity of the racket is best described as simply force.

Kinetic energy is a very significant component of the force which the racket posesses, but not responsible for all of it. More importantly, any kinetic energy that the racket posesses was imparted onto it by the body exerting force on the racket handle. Therefore even the amount of kinetic energy the racket attains is entirely dependent on the F = M * A equation (given optimal technique of course), and therefore the mass of the body. As for the rest of the racket's energy (that which hasn't been converted into kinetic energy), that comes in the form of elastic energy applied directly through the hand/arm/s holding on to its handle and by extention the player's body, as both I and Brian Gordon have explained, and I explained at great length in my last post. Therefore F = M * A ultimately determines both the kinetic energy and elastic energy that combined equal the racket's total energy.

Also I should point out that you contradict yourself in the first paragraph. At the beginning of it you say that the person's body is not a part of either the force or kinetic energy equation, but in the third sentence you say that "the mass of the person may or may not matter for accelerating the racket." But that's just a detail. Everything is explained in my previous paragraph. There are other problems with your post, such as your claim that the racket's velocity after contact is meaningless, when in fact the small amount of time that the racket is in contact with the ball (in reality there is no such thing as an instantaneous moment) is actually extremely important, as is the behavior (rate of deceleration) of the racket during this period.

[crass_lawner]

I personally don't mind continuing a debate even when things get contentious. If people always stopped debating at that point, then very little of interest would get debated. I find it unfortunate that both John and Brian Gordon say they have given up on this thread, as I don't think it was because of anything that I said, but rather things that someone else said. I also found much of what BG said to be useful and informative. I would be interested to hear what BG has to say about what I type here, but if he has truly given up on this thread due to things that someone else said, then so be it.

Actually I'm going to make two posts, the first of which is a response to a post that Brian Gordon made earlier. Actually the most encouraging thing I've read here was when you (BG) wrote:

The torque will over time (angular impulse) cause the body angular momentum to increase and it can then be transferred through the body to the racquet.

Here you are acknowledging the most fundamental point that I've been trying to make here, since what you are saying (in more scientific terms) is that body rotation allows one to put one's weight into the ball. This is pretty much the most fundamental physical phenomenon in the game of tennis and first John and now rosooki deny it's existence. And not only did John deny its existence, but also claimed that even if it did exist, it wouldn't be significant to understand to understand the physics of tennis.

So BG surely you must admit that transferring one's energy from one's body to one's racket not only occurs but is the most significant physical phenomenon in every tennis stroke? I'm sure you don't like the idea of contradicting John, but are you willing to deny scientific truth in order to avoid that? And I know you said you were done with this thread, but you said that because of things that airforce said, not because of anything that I said, so wouldn't it be worth acknowledging the truth here? By the way John, I'm sorry if the preponderance of supposedly smart people really are telling you that body mass doesn't matter for a tennis stroke, but just because the preponderance of supposedly smart people tell you something doesn't mean it's true. Especially in an area of study as nascent as tennis bio-mechanics.

Now that that issue should be settled, I just want to address something that is somewhat less of an issue. This is an area in which I disagreed somewhat with what BG said. You (BG) wrote that:

The transfer is accomplished via the joints using joint torques and forces and action/reaction. Assuming the that a heavier person is proportionally larger across the board then the transfer at the joints requires more joint torque and force to attain the same acceleration and angular acceleration at each segment in the chain. I believe this is why one typically sees athletes, for which the velocity of distal endpoint (racquet) is a priority, possess long/lean segments rather than massive ones.

As I said before, I realize that a heavier person "requires more joint torque and force to attain the same acceleration and angular acceleration at each segment in the chain," but my point is, according to the F = M * A equation, a heavier person doesn't even need to attain the same level of acceleration as a less heavy person in order to create the same force. This is why bigger players like Safin and Nalbandian make the game look so effortless at times. And of course, as I've mentioned many times here (and is obvious from the equation), it follows that if a heavier person were to achieve the same level of acceleration as a lighter person (which would obviously require them to use more strength), he would impart more force on the ball.

I must also point out an obvious flaw in the last sentence in the above quote. The velocity of distal endpoint plays pretty much the same role in baseball as it does in tennis. David Ortiz and Manny Ramirez don't exactly have "long/lean segments." As I said in one of my ealier posts (which I'm getting a sneaking suspicion don't get read in their entirety):

perhaps if the sport of tennis were just a matter of hitting the hardest two handed backhands that land in the court then most tennis players would look more like NFL linemen, but unfortunately for them a real tennis player must then use that same body to run back and forth and up and down on an asphalt court for hours on end, something that very heavy people would have trouble doing. And I would also guess that just hitting a pro-level two-handed backhand requires a level of precision, flexibility, timing, and balance that many NFL linemen would have trouble with.

Now I must address the last section of your (BG's) earlier post in which you wrote:

Therefore, heavier folks will need to be proportionally stronger and work harder to attain the same racquet speed. So… if it is your contention that more weight implies more strength and fitness to the extent that one can repeatedly overcome the inherent inertial resistance to attain greater acceleration for the body and at each link in the chain then your theory holds – this is not what you seem to be saying, however.

As I hope you can now tell, what you thought was my contention was actually not my contention at all, since a person with more weight will not have to attain greater acceleration than a person with less weight at all. But that should all be clear by what I wrote earlier.

[airforce1]

Actually, Crass_Lawner, the mass involved in both equations is the mass at the center of gravity of the moving object. In this case, that would be the point on the tennis racket where the ball strikes it, not the person's body, which is not part of the equation at all. It is the racket's mass that matters, not the mass of the person.

Seems you get to the heart of the issue here. You seem very sure that the mass of the racket is all that matters. What is your basis for this?

It seems what you say would be true for certain players and maybe certain swing types,
but it also seems that some players are able to connect and get some of their body weight involved in the cg of the moving mass. Are you suggesting that is impossible?


**Please take no offence, as I'm trying to ask simple questions on the issues. Apparently in print, things can be easily perceived as offensive.

[rosooki]

Mass of the Player is not an issue

Actually, Crass_Lawner, the mass involved in both equations is the mass at the center of gravity of the moving object. In this case, that would be the point on the tennis racket where the ball strikes it, not the person's body, which is not part of the equation at all. It is the racket's mass that matters, not the mass of the person. The mass of the person may or may not matter for accelerating the racket up to a given velocity depending on biomechanics, skill, strength, etc. but it is not a given that a fat (massive) guy of strength and skill x is going to hit the ball harder than a thin wiry guy of the same strength and skill. If it is not a given, it is not a rule.

Secondly, the racket may be accelerating through the shot, but there is a specific velocity that the racket hits the ball at at the moment it imparts its energy to it. That is the velocity we are interested in, not the velocity before the hit, or after the hit, or how fast (acceleration) that velocity is changing.

Ceterus Paribus (All other things being equal) - that's the magic sentence in dealing with holding down complexity in any science, velocity matters much more than mass. Yes, mass matters. Why? Because the ball we are trying to impart energy to has a specific set mass and our racket mass has to be set with that in mind. A supersonic toothpick won't be optimal. There will come a point where the mass is so small that increases in velocity would have to be huge to overcome the difficulties created. So given equal rackets, equal strings, equal arms, wrists, technique, strength, wind, margarine spread on the toast eaten for breakfast, etc. changes in velocity will yield greater changes on results than changes in mass.

As for my "flippant comment" my point was that one process you might follow would be to find the maximum swing speed your body is capable of in any given situation, then choose the heaviest stick you can swing at that speed - reliably and consistently. If you are really enthralled by mass, try the process the other way around and see how that works for you. My guess is that it won't.

[airforce1]

AF,

I doubt you and I are likely to ever come to agreement on any of this. Your interpretation of events and intentions are just 180 degrees from the way I see it. I'd be lying if I said they are not offensive. So I don't see a productive avenue for further dialogue. That happens sometimes. Onward.

Ok, that is fine. It's pretty clear, given this from the 3D study results-

"The goal values presented on the interface are my best estimate (at this time) of the most mechanically advantageous values for a given item. These values were initially set from composites (averages) of several Division I college players."

- that it has very questionable application to the question of "differences" between elite pro servers and avg. pro servers.

Not sure how anyone would be offended by that, as the study is probably excellent for it's intended uses.

Onward is fine.

[crass_lawner]

Thank you to both Brian Gordon and Rosooki for contributing your physics knowledge to this post.

Of course in the end, if you are able to move the heavier mass at the same velocity as the smaller mass, then good for you.

I thought you were finally going to make my point at the end of your post, but instead you just made a flippant comment like many others here have chosen to do. The main point I've been trying to make all along here is that if a heavier mass moves at the same rate of acceleration (or velocity) as a lighter mass, the heavier mass will contain more force (or kinetic energy) than the lighter mass will. I believe that this additional force is in the form of inertia.

In tennis terms this means that if a heavy player can swing the racket with the same amount of acceleration (and speed) as a lighter player, he will impart more force (and kinetic energy) onto the ball. Yes, this will take more effort, as BG pointed out earlier, but the effort will be rewarded with a ball that is struck harder. This applies with either the force or the kinetic energy equation, although weight is obviously less of a factor in the kinetic energy equation. This was the essence of what I've been trying to say all along here, but for some reason people are either unwilling to or unable to understand that.

And although I agree that kinetic energy is very important in tennis, I don't think it's the only relevant form of energy. There's also the amount of force that the player absorbs with his/her arm/s. Anyone who has ever suffered from tennis elbow can tell you that that is a pretty significant force. As you know this amount of force partially determines (along with kinetic energy) how much the racket will decelerate upon impact, which in turn is a factor in determining how much force will be imparted on the ball.

I think you would agree, especially if you've ever hit a 2 handed-backhand, that this force that is absorbed by the arm/s is significantly greater with the 2hbh, in which the non-dominant arm absorbs the brunt of the force, than with either the 1hbh or forehand. The increased energy absorption in the 2hbh is related to the shorter levers (arms) and slower racket speed/acceleration.

Since velocity isn't a factor, the amount of force the arm absorbs must be a function of the f = m * a equation, not the k = 1/2 m * a^2 equation. That's why body weight plays an even greater role in determining the force imparted on the ball with a 2hbh than with either a 1hbh or forehand. And, as I've said before, that's the main reason why the massive Nalbandian has such a great 2hbh.

Also, since mass is a factor in both of these equations (force and acceleration), when John said:

Intuitively working with players it's obvious the whole weight issue is a non factor, but proving it is beyond my expertise and I am afraid also that of the posters here.

... he was wrong.

[johnyandell]

AF,

I doubt you and I are likely to ever come to agreement on any of this. Your interpretation of events and intentions are just 180 degrees from the way I see it. I'd be lying if I said they are not offensive. So I don't see a productive avenue for further dialogue. That happens sometimes. Onward.

[airforce1]

John I'm more than happy to drop it with a smile, but no, my feelings were not hurt at all and my ego was never in the game. It's pretty amusing that you could ever see it that way, but I guess your great respect for him and his area of study tend to push your perspective that way (if you were serious with that comment, which I have my doubts).

Remember that I'm not the one touting my considerable accomplishments, but instead let the ideas I share here, stand on their own merit. BG's attempt to baffle with the use of a bunch of his industry terms only reflects on his needs. Clearly I'm not impressed by it, but I know many people are, so that is fine. If he was more confident of his position, he would handle that just the opposite and try to be understood with legitimate points and common language.

My only point at the beginning was to speak up for the very educated and accomplished readers we have on this site. He was the one who for some reason, inserted himself into this and announced he was offended (as though that were of interest to me). I would not have bothered to even answer him if he had not asked me questions directly that were full of holes and aimed to intimidate. One of my many personal flaws is that it is just too hard for me to step back from a bully.

I do have serious concerns if the 2 of you truly don't see the mis-application of his serve study group data collection, to the question of discerning differences between elite pro servers,
(defined as the ones very capable of going into the 130s at will- and yes, this includes Pistol Pete)
from the avg velocity pro servers.
There was never any mention by me of anyone "needing" a 135mph serve, so I don't understand that red herring.
BUT, where one is needed or not, there are a great number of players very interested in having one. Clearly Pete was THE elite server for me, and I often refer to him as the gold standard of serving. John, surely you recognize that Pete could go 135 anytime he wanted (and did), so why you would cite him as not of that group eludes me. I also never suggested his study was invalid, just not applicable. Where do these things come from?
And Pete's data was NOt in the data when we discussed this before. Has it been added since? If so, admit that.
These are exactly the type of errors I'm citing on the topic.

I can discuss the merits on each side of this without being offended or casting obvious insults. I apologize that I can't be sure to avoid casting slights that are unintended or imagined. I can also understand that if you don't want to deal with it, as I don't see where there is any point to be made for your side of things on this. That is not an insult, but just a statement of assessment. I would actually like to be wrong on this, because it is not my ego on the line here, but the desire for more useful info in this area where I have great interest going back over a decade now. Hopefully you and BG can show me how to apply his work to the question posed, because contrary to what he imagines, I have great respect for his work and his ability to do it. I just don't care for his customer relations, and am surprised you allow him to post unedited. It would probably help his career and I am all for that. Not everyone can pull off that "Dr. House" personality and stay viable.

And besides his personal attacks at me, I think we are very fortunate to have someone with his interest and drive, working on these tennis issues. If you look back you will see I have been very complimentary of his efforts, and only gotten sideways with him when he asked for it. I will pledge to not take his bait in the future, regardless of how inviting, if you 2 will address the above concern in a reasonable fashion.
Are you up to handling this like gentlemen?

[johnyandell]

Well, even a blind pig finds a truffle once in a while. Interesting explanation, Rosooki, and if that is indeed the case (again remember I got a B in high school physics in 1970 and the teacher was our football coach), it fits with my experience of racket weight and swing speed--how average players usually hit the ball harder with say 11oz rackets.

AF:

I sincerely doubt that if Brian's data base was the top 100 pro players instead of elite college players that there would be some revelation that invalidates his work.

When you say you need a 135mph serve that's actually preposterous. Unless you think Roger Federer can't serve. And Pete Sampras actually averaged say 125mph. But maybe you don't think his serve wouldn't work in the current game.

Brian is too modest (humble?) to go into it, but he and I also did the first 3D study of Pete's serve and will be releasing that data over time. What I think it will show is that all the factors he idenitifed in that landmark series on this site are applicable for studying and understanding Pete--who sorry still probably can claim the greatest serve in history. It's a new framework for understanding how the body works--and god willing he and I will continue to make it more and more comprehensible.

As for college players slinging a racket for a mishit--as if that was what his data base entails--again preposterous.

I was there when Brian filmed players from national championship teams from Georgia and now, Southern California. Uh, those guys are good players. Some of them will make the top two hundred or higher at some point.

Which elite players are you working with that you feel you need 135mph models? Have a dozen or so that have played at a higher level than the elite college players Brian described? Or maybe someone with a win over Pete?

Let's be honest here--you and he traded punches over on the TW board and you got your feelings hurt and your ego bruised. I know because I've been in the same space.

Best to just let it all chill out and forget it. Heath Waters claimed he was ready to come over to my house and try to shoot me at one point, but, luckily, we both realized we were being asses and decided to have a laugh and drop it.

[rosooki]

Not F = MA

Actually F=MA is the wrong equation to be focusing on here. That equation will tell you how much force you need to accelerate a given mass at a given acceleration rate. It is the equation to look at when we're talking about bringing the racket up to speed from a starting point.

The better equation to focus on for this issue is K=1/2mv^2, which is derivable from F=MA. This is kinetic energy or the energy of a body in motion. It is the energy imparted to the ball when it is hit by the racket. Looking at this equation shows why John Yandell believes velocity is so important. the V component is squared. So any change in it leads to squared changes in kinetic energy versus linear changes to total energy from raising mass. That's why tiny speed raises in your car lead to much longer stopping distances - much more energy (-K) is needed to stop your car.

It also says that if you want to hit home runs, you want a faster bat, more than you want a bigger bat. If you want faster groundstrokes you want a faster racket, more than a bigger racket. Of course in the end, if you are able to move the heavier mass at the same velocity as the smaller mass, then good for you.