The Forehand: Busting misconceptions once and for all

[TCF]

==========================

[10isfreak]

Originally Posted by dominikk1985

Regarding the sliding on the strings:

yes the contact time is short but the Speed is also high.
a 100+ mph Forehand with a lot of spin has at least 80 mph RHS.

that is 35 m/s. asuming that the contact time is 3/1000 of a second the racket moves 35/1000*3=0.105m or about 4 inches.

of course not all of this is up since the swing is also Forward but it is not unreasonable that about a third of it (33% upswing angle) is upward which would mean that the racket moves about 1.5 inches up during contact time.

To give credence to your estimation, let's validate the approximation. Most pros hit at an angle less than 20 degrees and usually higher than 10 degrees (in terms of slope). We could deal with this using the tangent and a 100 units long side in a rectangular triangle.

Tan(20) = x/100 which leads us to 100 tan(20) = x = 36,4
Tan(10) = x/100 leads us to x =17,6

So, between 17,6 percent and 36,4 percent of the vector is oriented upward. With 35 m/s, it's 6,17 to 12,74 mm of upward movement.

We may have a good case to think that, indeed, the ball might move... An alternative possibility is that the ball moves along with the racket during this dwell time; yet an other possibility is that of a combination of sliding ball and moving ball. If we believe SpeedMaster's analysis (that much of spin is generated through the racket tilt and a contact bellow center -- and by bellow, I mean closer to the ground, not the throat), only a fraction of this sliding effect would contribute to spin and this fraction should be less 50%, to say the very least.

A good way of finding if someone is educated or not is asking this person if things like this are complicated or not to understand. If he answers the later, he's either a genius who didn't reveal his conclusions or he sees the tip of the iceberg.

[sureshs]

Originally Posted by TCF

Effective mass is the mass of the racquet 'effectively' involved in a hit at a given location of the stringbed. The entire mass of the racquet is not involved during this collision, and no part of the forehand is included in effective mass.

I think you are right in this context of the TWU prof.

I was thinking of something else I had read when people do momentum calculations of the racket hitting the ball and want to know what mass to use. This case probably refers to only a part of the racket.

[sureshs]

Originally Posted by 10isfreak

To give credence to your estimation, let's validate the approximation. Most pros hit at an angle less than 20 degrees and usually higher than 10 degrees (in terms of slope). We could deal with this using the tangent and a 100 units long side in a rectangular triangle.

Tan(20) = x/100 which leads us to 100 tan(20) = x = 36,4
Tan(10) = x/100 leads us to x =17,6

So, between 17,6 percent and 36,4 percent of the vector is oriented upward. With 35 m/s, it's 6,17 to 12,74 mm of upward movement.

We may have a good case to think that, indeed, the ball might move... An alternative possibility is that the ball moves along with the racket during this dwell time; yet an other possibility is that of a combination of sliding ball and moving ball. If we believe SpeedMaster's analysis (that much of spin is generated through the racket tilt and a contact bellow center -- and by bellow, I mean closer to the ground, not the throat), only a fraction of this sliding effect would contribute to spin and this fraction should be less 50%, to say the very least.

A good way of finding if someone is educated or not is asking this person if things like this are complicated or not to understand. If he answers the later, he's either a genius who didn't reveal his conclusions or he sees the tip of the iceberg.

So I think what you saying is the ball is carried by the string some distance. In fact along the target arc, it might get carried more than two ball widths, and the upward part of that is probably what you are calculating. Along with that, the ball also slides along the strings a little.

[Relinquis]

wow... if you guys spent half the time you spend posting on this single thread on tennis courts you would have amazing forehands...

good discussion though. maybe some of you should pursue a tennis physics phd or something.

[sureshs]

Originally Posted by Relinquis

wow... if you guys spent half the time you spend posting on this single thread on tennis courts you would have amazing forehands...

good discussion though. maybe some of you should pursue a tennis physics phd or something.

It is called multitasking. The posting time does not come out of tennis time.

[sureshs]

Originally Posted by Wegner

Suresh, perhaps you'd like to start a thread on the serve. I'll be happy to participate. I am in the middle of moving, but next week I'll have more time.

Is it moving or footwork

New thread:

http://tt.tennis-warehouse.com/showthread.php?t=451401

[dominikk1985]

Originally Posted by 10isfreak

To give credence to your estimation, let's validate the approximation. Most pros hit at an angle less than 20 degrees and usually higher than 10 degrees (in terms of slope). We could deal with this using the tangent and a 100 units long side in a rectangular triangle.

Tan(20) = x/100 which leads us to 100 tan(20) = x = 36,4
Tan(10) = x/100 leads us to x =17,6

So, between 17,6 percent and 36,4 percent of the vector is oriented upward. With 35 m/s, it's 6,17 to 12,74 mm of upward movement.

We may have a good case to think that, indeed, the ball might move... An alternative possibility is that the ball moves along with the racket during this dwell time; yet an other possibility is that of a combination of sliding ball and moving ball. If we believe SpeedMaster's analysis (that much of spin is generated through the racket tilt and a contact bellow center -- and by bellow, I mean closer to the ground, not the throat), only a fraction of this sliding effect would contribute to spin and this fraction should be less 50%, to say the very least.

A good way of finding if someone is educated or not is asking this person if things like this are complicated or not to understand. If he answers the later, he's either a genius who didn't reveal his conclusions or he sees the tip of the iceberg.

I have seen that 18 degree number too but I think that is mostly on balls hit on chest Level. on balls below waist the upward swing angle is probably more than 30 degrees.

when fed hits a low ball he will swing up much more. the upward angle is not only to create spin but also trajectory. to hit a ball at the knees with 3 feet net clearance you Need quite a bit of lift.

here is a nadal FH on a low ball with more than 45 degrees upward swing
http://www.youtube.com/watch?v=NSr6dfxhMUw

and here are higher balls with a flatter swing (more like 25 degree I estimate)
http://www.youtube.com/watch?v=NSr6dfxhMUw

and on chest high balls he will swing probably even flatter. swing trajectory has a Lot to do with strike height.

[Povl Carstensen]

Originally Posted by dominikk1985

I have seen that 18 degree number too but I think that is mostly on balls hit on chest Level. on balls below waist the upward swing angle is probably more than 30 degrees.

when fed hits a low ball he will swing up much more. the upward angle is not only to create spin but also trajectory. to hit a ball at the knees with 3 feet net clearance you Need quite a bit of lift.

here is a nadal FH on a low ball with more than 45 degrees upward swing
http://www.youtube.com/watch?v=NSr6dfxhMUw

and here are higher balls with a flatter swing (more like 25 degree I estimate)
http://www.youtube.com/watch?v=NSr6dfxhMUw

and on chest high balls he will swing probably even flatter. swing trajectory has a Lot to do with strike height.

Subtle difference between the low and high balls...

[julian]

Originally Posted by toly

This is just fancy video and no statement so far, but I will collect some more data and give you particular statement.
Btw, what is effective mass?

What is your statement,please?

[corners]

Originally Posted by julian

Some people have to be patient
I do NOT have a laptop at a tennis club I work.
I try to post from a cell -it is a pain.
Cut and past is complex.
Incremental savings are necessary and a screen is small
regards,
Julian

OK, got it.

[corners]

Originally Posted by toly

No, it is not.
There is citation from Rod Cross article, PHYSICS OF TENNIS http://www.physics.usyd.edu.au/~cross/tennis.html,

“4. The dead spot of a racquet

Clamp the end of the handle on a table, using your hand to press on the handle, so the rest of the racquet hangs over the edge of the table. Then drop a ball onto the strings at various points. The ball will bounce best near the throat. There is a spot near the tip where the ball doesn't bounce at all. That's the dead spot. At the dead spot, all of the energy of the ball is given to the racquet, and the racquet does not give any energy back to the ball.

The reason is that the effective mass of the racquet at that point is equal to the mass of the ball. The effective mass is the ratio of the force at that point to the acceleration at that point (F = ma so m = F/a). If a ball of mass m collides head-on with another ball of mass m at rest, then the incident ball stops dead and gives all its energy to the other ball.“

This racquet’s property is driving me crazy.

Here is a tool that shows the effective mass (also called "hittingweight") at various impact locations for most racquets produced in the past five years: http://twu.tennis-warehouse.com/cgi-...ttingwtrac.cgi You can open up several windows to compare racquets.

Hittingweight is pretty much proportional to swingweight for impacts along the longitudinal axis, with balance point and static mass playing only a small role. Twistweight (or polar moment of inertia) becomes a factor for impacts to either side of the long axis. The hittingweight at the balance point equals the static mass of the racquet and decreases the further away from the balance point you go. So it's tiny at the tip and butt of the racquet.

And here is a tool at USRSA that lets you play with swingweight, balance and static mass and see the effective mass along the longitudinal axis: http://www.racquettech.com/store/lea...ctivemass.html

And this USRSA tool adds twistweight, allowing calculation of impacts at locations not on the longitudinal axis: http://www.racquettech.com/store/lea...ssgeneral.html

Also, if you spend much time reading old articles by Cross, Brody and other physicists studying racquets you'll run into lots of estimations of apparent coefficient of restitution (ACOR). These articles were all written prior to the creation of Tennis Warehouse University, where nearly every racquet produced in the past five years has had its ACOR measured experimentally, rather than estimated mathematically. This is probably the most useful tool to access the ACOR ("Power Potential") database, as it allows a comparison between two racquets with the specs of those racquets clearly listed: http://twu.tennis-warehouse.com/lear...r/contours.php ACOR is pretty much proportional to hitting weight, with some minor role played by twistweight on off-center impacts, stiffness on impacts near the tip of the racquet. String pattern has a general, but small, effect on ACOR.

[julian]

Originally Posted by 10isfreak

Many of you might have encountered the tennisspeed blog. In there, we do not get served the usual beliefs we hear during all too many lessons... we are greated with hard science, with a big pile of evidence and repports that overviews some beliefs regarding tennis. The question is whether they are accurate in part, as a whole or if they are simply wrong.

I will just give you 3 key elements that he overviewed among others.

Your swing path prior contact mostly controls the ball's launching angle -- i.e., ceteris paribus, the more vertical your swing path, the higher the ball flies before starting to fall. As you can read, it's not the most determining factor in generating top spin.

The racket head is not perfectly vertical at contact, but is ideally tilted forward. Why? Because spin is about accelerating the edge of the ball and most of your energy is directed forward... so, trying to accelerate the upper edge of the ball should, in principle, capitalize on a lot of energy compared to trying to accelerate upward the edge that is behind the ball.

There is a direct relationship between spin production and how low the ball makes contact with the string bed. The racket is nearly horizontal during ground strokes, so low actually means near the side which is closest to the ground, not near the throat -- just to clear up the potential confusion. A low contact ensures the highest spin/pace ratio (that is, you get more spin, less pace this way).

Theoretical material and concepts are not purely useless, abstract things... If you actually know what pros do and how they do it, you're one step closer to doing it yourself: you just need to figure out how to incorporate these into your swing. From that point and on, you can go on the court knowing that you have a factually valid answer that is guaranteed to yield results. AND YOU GOT IT FOR FREE!

10isfreak,

The thread went a wrong direction
The only interesting aspect is a technique to generate a VERTICAL component of forehand AND some progressions related to it
IMHO only three websites provide some info on the subject of the vertical component
1.mention above
2.Heath Waters
3.John Yandell
Positioning of a knuckle and pronation/supination related issues can be discussed.
http://www.youtube.com/watch?v=NSr6dfxhMUw is probably a good starting point
More or less the conversation is related to what John Yandell and Brian Gordon and Macci describe as "flip"
If you want to get in touch with me via E-mail you are welcome to do so
Regards,
Julian Mielniczuk
PS #1
A vertical component of forehand achieves a HALF of the value of a HORIZONTAL component
in 1/20 of time the HORIZONTAL component achieves the max value
It is explains some problems with consistency/shanking/wrist injuries etc

[corners]

Originally Posted by 10isfreak

In the first place, I never intended to mean that Yandell was addressing this question. I meant that people were using his results improperly.

Secondly, the claim regarding the contact location doesn't rely on a ball's size spot on the string bed. The effect I am looking to get is a fast closure of the racket face upon contact. Any ball struck bellow the geometric center will make it happen to some extent -- that is, you have half a racket string bed to manage getting it right.

The study the SpeedMaster did divided contacts in terms of racket reaction after impact: stable, closed, opened. That is sweet spot, upper half or lower half. You can err a bit toward the tip, the edge or the throat... it still twists the racket. Maybe I worded it poorly by phrasing it the way I did. In my head, it was very clear that relationship wasn't continuous in the sense that each fraction of an inch would give you more spin, but looking at the sentences I wrote, I wasn't accurate enough.

I did provide a chart with three zones, but it's not clear how to use it. To me, you don't use a marginal analysis to make inferences given qualitative data...

OK, I see what you're saying. It would make sense that any hit below center would generate more spin, if we assume a gradient from 2" from center, where 30+% more spin is generated, compared to the center. So, possibly, impacts 1" from center might generate 15% more spin then center impacts. There might be a linear spin gradient from the edge toward the center, in other words. But this is assuming a lot, as the stringbed effects responsible for that extra spin are complex and some of them rely on close proximity to the frame edge.

The rotation effect you mention is one of those effects, but given the physics of racquet rotational instability - twisting moments are much greater the further the impact is from center - one would think that the rotation effect would be much less effective at producing extra spin on impacts 1" from the center than it would be at 2" from the center. But I think another experiment would be needed to sort that out.

For those who are wondering what experiments I'm talking about: http://twu.tennis-warehouse.com/lear...r/location.php

The rotation effect can also be applied by the player if he swings so that the top edge of the racquet is accelerating relative to the bottom edge at contact. This will apply a relatively larger force to the top of the ball than the bottom of the ball, producing more spin, even on impacts at locations other than below center and without the racquet spinning in reaction to the ball impact. Rod Cross gives a good explanation of this in a paper on the physics of the kick serve, see Part II, Section 5. Racquet Rotation here: http://twu.tennis-warehouse.com/lear...r/location.php

So there are several racquet rotation events that can generate extra spin:

1. Impacts below the center of the strings produce more spin, all other things equal, due to various stringbed phenomena, one of which is a result of the off-axis impact causing the racquet to "close", which results in greater force being applied to the "top" of the ball.

2. The player can swing in such a way that the racquet-face is closing during impact. In other words, just before impact it is less closed than during and after impact. This will have a similar effect as above, but is not necessarily dependent on impact location. A player could do this by actively pronating the forearm to close the racquet face through impact. I believe coaches sometimes refer to this as "grinding" the ball to produce additional spin at impact. However, there is not enough time during impact for a player to decide, at that point, to do this. The rotation of the racquet would have to be set in motion prior to contact. (Although this could probably be debated by talking about reflex contractions of the forearm muscles, etc., but I'm not sure it would lead anywhere productive.)

3. Racquet tilt should also be considered along with the above. If a racquet is "closed" during the swing, and that angle remains constant so that it is not rotating during contact as in #2 above, it will still produce more spin for the same physical reason as the two rotation events - the closed racquet face will apply greater force to the top of the ball than the bottom of the ball.


I don't know, but I still think it would be better to aim for the center of the strings, or at least along the longitudinal axis. This would seem to give the greatest chance of producing a good shot, although shot speed and spin will vary each time one misses the center, while reducing the chance of clipping the frame and losing the point outright.

[toly]

Originally Posted by corners

I don't know, but I still think it would be better to aim for the center of the strings, or at least along the longitudinal axis. This would seem to give the greatest chance of producing a good shot, although shot speed and spin will vary each time one misses the center, while reducing the chance of clipping the frame and losing the point outright.

I think you are right and thank you for post #192.

Below is set of pictures that show point of contact. They copied from random samplings of following high speed videos:

1. Federer Inside Out Forehand tennisplayer.net https://www.youtube.com/watch?v=z6LmJyNoL8U
2. Sharapova Inside Out Forehand1 http://www.youtube.com/watch?v=uC254i1XN9I
3. AO 2012 Sharapova Easy Short Ball Forehand Winner http://www.youtube.com/watch?v=n9egjkaBnYg
4. AO 2012 Kvitova Forehand Return Serve Winner http://www.youtube.com/watch?v=Bnezy_gBxpY
5. AO 2012 Kvitova Second Serve Return Forehand http://www.youtube.com/watch?v=Z8xrhvDIE50
6. AO 2012 Nadal Down the Line Running Forehand http://www.youtube.com/watch?v=594rg_Lrcjw
7. AO 2012 Djokovic Baseline Forehand http://www.youtube.com/watch?v=f5jWEzDsxWg
8. AO 2012 Djokovic Short Ball Forehand http://www.youtube.com/watch?v=VrEwPvhT3lU
9. AO 2012 Djokovic Down the Line Backhand Winner http://www.youtube.com/watch?v=zlDsdMOXnHw
10. AO 2012 Djokovic First Serve Forehand Return Winner http://www.youtube.com/watch?v=eMECK-D9Dqk
11. Lleyton Hewitt Forehand http://www.youtube.com/watch?v=P4jgaPbHcZY
12. Davydenko Forehand http://youtu.be/hF4dlqFH_v8



IMO the best players don’t have tendency to hit ball below of the racquet longitude axis!!!

[sureshs]

The counter argument will be that they wished they had hit below center, but couldn't. Desire and reality, cause and effect, are so intertwined that we will never know.

[corners]

Originally Posted by sureshs

The counter argument will be that they wished they had hit below center, but couldn't. Desire and reality, cause and effect, are so intertwined that we will never know.

I wouldn't buy that counter argument. If they are aiming there they will hit there more than they will hit other locations. To think otherwise is preposterous: All these top players, all trying in vain to strike the ball below the center of the stringbed, and failing, continually, year after year....

Just think, if they could only manage to hit below center consistently, how good they would be!

Besides, if we wanted to know if they are aiming there or not we could just ask them.

[tommyfr]

What a sample, 12 pictures and all middle or above middle!

I have seen 100s of videos and pictures, in my estimate about 50 % are below longitude midline. So I think this sample is not at all closely to being random....very selective...Cheating, in short. Argumentation the Ugly way.

[dominikk1985]

I asked "our" pro filip peliwo about that.

he said he thinks that he usually hits with a slightly closed face (but is not sure because he doesn't have slow mo-we don't know if he is experiencing the closing after contact or actual contact since that happens so fast), but he is pretty sure that he doesn't hit the ball below Center. of course this is just an anecdote because he is not a scientist and many pros actually don't know what they are doing in Detail since that happens so fast and they do it intuitively since their childhood (a lot of MLB Players think they swing down to contact) but I think it's still interesting to read.

Originally Posted by fpeliwo

It's tough to say, I can't speak for myself cause I've never seen a close up slow motion of my forehand. Fed seems to hit the ball down there quite often. He does have quite a small racket head so that doesn't make it easier to tell. I definitely don't hit the ball down there though, all my shots are towards the upper beam and top of the head, so towards the far top corner. If I hit the ball that far down my frame it would be a shank. I guess it depends on the player. You have to remember Federer has a grip tilted very much towards an Eastern or even towards a Continental grip so his forehand mechanics and strike zone won't be the same as someone who uses a semi-western or western grip. As for the tilting, I don't think I hit with a completely flat racket face (Unless I hit a flat shot), it is always a bit closed on contact, but only minimally, if anything. Wouldn't be able to tell you for certain until I see a slow mo of my fh.

[toly]

Originally Posted by tommyfr

What a sample, 12 pictures and all middle or above middle!

I have seen 100s of videos and pictures, in my estimate about 50 % are below longitude midline. So I think this sample is not at all closely to being random....very selective...Cheating, in short. Argumentation the Ugly way.

Since the likelihood that the ball just hits the racquet longitude axis is almost zero, thus according to your data 50% of ball contacts would be above middle line and 50% below. So, your data also demonstrate that players don’t have tendency hitting the ball below middle line.::

Some of my pictures (1, 4, 7, 10) show impact below middle line.