Trouble with Eastern Forehand

[10isfreak]

Originally Posted by rkelley

If you want the big topspin, the racquet has to have a large upward component to its velocity that is perpendicular to the ball. That's just physics - like you said swing up.

That's not physics actually. What you depict here is a major misunderstanding.

Spin is the edge of the ball moving relative to its center. Let's simplify the problem for you to grasp it. Let's say the ball is only two-dimensional (it's a circle). If you apply a vector is directly forward-directed to the upper edge, what happens? The edge moves forward, which is the same as saying that the edge which is bellow moves backward, which is the same as saying that the front edge moves downward and that the back edge moves upward. In essence, in front, bellow, behind or over... it doesn't matter, all of them can produce spin. That's physics. You do not need to have a vertical force to generate top spin because top spin is not directed upward. Our real possibilities are hitting the upper edge or bushing the back of the ball.

Unless you swing at more 45 degrees from an horizontal plane prior contact, most of your swing is directed forward... With a greater amount of force directed forward, it seems logical to assume that accelerating the upper edge is a better strategy.

[rkelley]

Originally Posted by 10isfreak

That's not physics actually. What you depict here is a major misunderstanding.

Let's see.

Originally Posted by 10isfreak

Spin is the edge of the ball moving relative to its center.

Agreed.

Originally Posted by 10isfreak

Let's simplify the problem for you to grasp it.

Was that comment really necessary?

Originally Posted by 10isfreak

Let's say the ball is only two-dimensional (it's a circle). If you apply a vector is directly forward-directed to the upper edge, what happens? The edge moves forward, which is the same as saying that the edge which is bellow moves backward . . .

Agreed. But how do you apply a (force) vector to the upper edge of the ball that has a direction that is "forward-directed?" In the case you putting forth the force vector is not in line with the center of the ball, correct? The ball's center of mass is at its geometric center, so the vector you're describing creates a moment around the center of the ball that creates spin, correct?

But a tennis racquet is a plane that interacts with the ball. The point where that plane contacts the ball must always be normal to the surface of the ball. The reaction force from that contact has to go through the ball's center of mass. So using a plane to apply the force, how do I create the moment about the ball's center of mass?

Originally Posted by 10isfreak

You do not need to have a vertical force to generate top spin because top spin is not directed upward.

To be specific, you need a vertical velocity component of the racquet's swing path to generate topspin. Force alone, at least on the racquet, won't do it. You can put all the force you want on the racquet. If there hasn't been enough time for that force to sufficiently accelerate the racquet upward to create that significant vertical velocity component you're not going to get much topspin.

Originally Posted by 10isfreak

Our real possibilities are hitting the upper edge or bushing the back of the ball.

So how can I hit the upper edge of the ball with a plane (the racquet) without just directing the ball down? Brushing the back of the ball is exactly what I'm talking about.

Originally Posted by 10isfreak

Unless you swing at more 45 degrees from an horizontal plane prior contact, most of your swing is directed forward... With a greater amount of force directed forward, it seems logical to assume that accelerating the upper edge is a better strategy.

Here's a couple of videos of Fed and Djokovic. They actually are swinging up at around 45°.

http://www.youtube.com/watch?v=1ImeQaAyFPc

http://www.youtube.com/watch?v=Me1tzm1nnWk

[LeeD]

er.. no.
I thought about that before the video loaded.
Swing is maybe 35 degrees upwards, as little as 30 degrees.
He's hitting a rallyball, to sustain the rally, against a deep ball, and he's going for lots of topspin to control depth and keep the opponent back.
Now, "almost 45 degrees" is realistic, but NOT the same as "up around 45 degrees".

[rkelley]

Originally Posted by LeeD

er.. no.
I thought about that before the video loaded.
Swing is maybe 35 degrees upwards, as little as 30 degrees.
He's hitting a rallyball, to sustain the rally, against a deep ball, and he's going for lots of topspin to control depth and keep the opponent back.
Now, "almost 45 degrees" is realistic, but NOT the same as "up around 45 degrees".

The basic point is that there is a significant upward component to the swing to generate the topspin that is seen in the modern game. It's not subtle, and it's not generated by closing the racquet face. The players are swinging the racquet up at very steep angles and very high racquet head speeds. It's not rocket science.

Here's Fed in a match ripping one, and the angle is pretty steep. I'll let Lee determine the exact measurement.

http://www.youtube.com/watch?v=kmhvK...e=results_main

[LeeD]

Now THAT one is a hair stronger than 45 degrees.
Talking rackethead path, of course.

[sureshs]

Originally Posted by rkelley

But a tennis racquet is a plane that interacts with the ball. The point where that plane contacts the ball must always be normal to the surface of the ball. The reaction force from that contact has to go through the ball's center of mass. So using a plane to apply the force, how do I create the moment about the ball's center of mass?

Not sure if this is most relevant, but this was put up a couple of days ago:

From TWU:

Reaction force components of racquet on ball. For reasons discussed below, the normal force is typically directed in front of the ball's center of mass.

The perpendicular rebound force usually does not act directly through the center of mass, but rather, in front of it. This happens whether or not the strings move laterally. That is because the ball starts to spin off of the strings on its back side and into the strings in front. The ball thus exerts greater force on the strings on its leading side. Friction slows the bottom half of the ball, causing the upper half to stretch and deform forward, shifting weight as it does so. The greater force on the strings at the front of the ball causes the net normal force to shift forward in front of the CM by a distance D — usually about 2-9 mm, depending on the stringbed setup. This distance, D, is often used to designate the normal force offset as "D-offset."


http://twu.tennis-warehouse.com/lear...inpatterns.php

[sureshs]

Originally Posted by 10isfreak

Most pros swing on a nearly horizontal plane prior contact.

I see their rackets going up a slope. Why do you say it is a nearly horizontal plane?

[sureshs]

Excerpts from my posts on another thread:


That is because most of the energy is in the forward direction with the up and across component coming from forces below the center of mass, and friction with the strings, which create the spin.

One of them is like flicking a vertical wheel on the lower side with an upwards and forwards force. As long as the line of force does not pass through the center of mass (axle) but above it, the wheel will rotate. Another way is to grip the wheel and turn it. These two are analogous to the strike on the ball followed by friction during the dwell time, and both require the across and up component.

If the direction of the force (and hence the swing just prior to it) was merely normal to the ball, it would just go straight. If the across and up direction passed through the center of mass, the ball would also be launched across and up. So it is a combination of the force in the forward direction which also acts up and across in a glancing way, which makes the combination of pace and spin possible.

Continuing on this topic, there seems to be a lack of appreciation of the dwell time and string friction, which is the basis for all the different varieties of strings and pro preferences etc.

Let us take a billiard ball and a cue. How would you impart some rotation to the ball? By hitting it off center so that the line of force does not pass through the center. You won't get much rotation. How would you increase it? You would have to make the contact more grazing or tangential. That is the only way.

In tennis, the ball and strings deform much more, and there is plenty of friction in the dwell time. This gives an opportunity to be less grazing, but still produce spin by an off center hit. The single arc can create both pace and spin without the need for the arc to be very steep. That is the kind of arc used most frequently.

Table tennis is somewhat in between.

[10isfreak]

[quote=rkelley;7182728]They actually are swinging up at around 45°.

Here's an analysis of their actual forehands prior contact:

[10isfreak]

Originally Posted by sureshs

I see their rackets going up a slope. Why do you say it is a nearly horizontal plane?

Less than 20 degrees is visually closer to an horizontal plane... so, to break with the common knowledge which advocates "low to high" and the impression we have of players hitting very vertically, I use this wording.

They do hit low to high... but it's a very shallow slope for most forehands. Federer is very consistently hitting on something around 16 degrees, unless he purposefully moonballs.

[10isfreak]

"To be specific, you need a vertical velocity component of the racquet's swing path to generate topspin. Force alone, at least on the racquet, won't do it. You can put all the force you want on the racquet. If there hasn't been enough time for that force to sufficiently accelerate the racquet upward to create that significant vertical velocity component you're not going to get much topspin."

Physically speaking, the top edge moving forward is also the back edge moving upward.
You do not need, as a matter of FACT, the racket to move upward in any way to create top spin, even if it does.

Top spin is also forward spin; it's also downward spin; it's also backward spin... it depends on your point of reference on the ball. In truth, they're not different: they're all the same thing happening at once.

Now, if you hit the ball slightly behind, all the while being above center, both upward and forward action creates top spin. That's what it is, period. You can start trying to spare your convictions as much as you want... I never said that it doesn't create spin -- I said that additional upward movement affects top spin marginally, for various reasons.

As for your impression regarding videos, all pros present a very vertical action once the contact is already made.

[sureshs]

[quote=10isfreak;7184514]

Originally Posted by rkelley

They actually are swinging up at around 45°.

Here's an analysis of their actual forehands prior contact:

I see the confusion. I would consider it as much more - the angle which the racket face makes with the horizontal. That is the angle "seen" by the ball. The lead-up slope to the ball is not important to the ball.

The angle with the normal can be 10 to 20 degrees, so with the ground, 70 to 80 degrees. You are looking at the swing path of the eventual point of impact.

PS: Perhaps the lead-up slope is important too because that puts the upward force on the ball. Need to think more about it.

[Dimcorner]

I think technically speaking you do not need upward force to create spin. If you hit with more forward force on the top of the ball vs the bottom of the ball then the net effect would be that the ball rotates with the leading edge being the side that was hit with more force.

Also I guess you can consider the timing. The top hits first, starts to rotate and at the same time might roll down a little (or squash down) on the racquet. By the time the back of the ball impacts the string bed the top might be already on it's way out (rebound and rotation since the bottom might still be in compression stage). That would mean that to top is already rotating the ball and leaving the racquet so the bottom of the ball contribution would be less right?

???

[Dimcorner]

I don't think you would spin the ball much this way, but I guess it's possible to spin it some.

[sureshs]

What this hints to me is that the brushing up action is less important for top spin than hitting through the ball in the arc with a somewhat closed face (which creates top spin due to the off-center orientation of the force of the racket on the ball). In other words, less "swiping" and more "hitting through."

If all this is correct of course .........

[sureshs]

Originally Posted by Dimcorner

I think technically speaking you do not need upward force to create spin. If you hit with more forward force on the top of the ball vs the bottom of the ball then the net effect would be that the ball rotates with the leading edge being the side that was hit with more force.

That is how top spin is modeled by TWU professor. The contact area is modeled as having two springs, one at the top and one at the bottom. If the top spring is compressed more than the bottom spring, top spin is produced. Vice versa produces slice.

[10isfreak]

Originally Posted by sureshs

What this hints to me is that the brushing up action is less important for top spin than hitting through the ball in the arc with a somewhat closed face (which creates top spin due to the off-center orientation of the force of the racket on the ball). In other words, less "swiping" and more "hitting through."

If all this is correct of course .........

Well, in theory, that's supposed to be what happens. Now, if we neglected something, it won't work. However, since pros so consistently show a slightly closed racket face, I would be confident that we nailed it.

[10isfreak]

[quote=sureshs;7184591]

Originally Posted by 10isfreak

The angle with the normal can be 10 to 20 degrees, so with the ground, 70 to 80 degrees. You are looking at the swing path of the eventual point of impact.

Well, the horizontal line in the drawing is parallel to the ground. If that model turns out to be true, following the advice of many coaches who ask you to brush up for more spin and meet the ball with a square (vertical) face would result in sending the ball higher (the vertical swing path) and being left with too little spin (the vertical racket face).

To my sense, it's what seems to happen to many amateurs.

[rkelley]

Originally Posted by 10isfreak

"To be specific, you need a vertical velocity component of the racquet's swing path to generate topspin. Force alone, at least on the racquet, won't do it. You can put all the force you want on the racquet. If there hasn't been enough time for that force to sufficiently accelerate the racquet upward to create that significant vertical velocity component you're not going to get much topspin."

Physically speaking, the top edge moving forward is also the back edge moving upward.
You do not need, as a matter of FACT, the racket to move upward in any way to create top spin, even if it does.

Top spin is also forward spin; it's also downward spin; it's also backward spin... it depends on your point of reference on the ball. In truth, they're not different: they're all the same thing happening at once.

Now, if you hit the ball slightly behind, all the while being above center, both upward and forward action creates top spin. That's what it is, period. You can start trying to spare your convictions as much as you want... I never said that it doesn't create spin -- I said that additional upward movement affects top spin marginally, for various reasons.

As for your impression regarding videos, all pros present a very vertical action once the contact is already made.

Wow, I missed all the bold and red.

I guess we disagree. That's cool.