Hitting up vs. Brushing up on the serve

[sureshs]

The racquet and strings aren’t moving upward between frames 2 and 3, right? Thus, there cannot be “decent amount of topspin” unless strings significantly slide downward and then back to original position during impact.

2 is before the apex, 3 is after the apex. What ever has to happen happened in this interesting time. The strings have both come under and over the ball in this time by deforming, building up the spin.

 

[bhupaes]

The racquet and strings aren’t moving upward between frames 2 and 3, right? Thus, there cannot be “decent amount of topspin” unless strings significantly slide downward and then back to original position during impact.

Toly, I am basing my statement on the well known fact that the first serve of any pro has a significant amount of topspin, but not as much as in their second serves. It is not evident from the images - you are right about that.

 

[sureshs]

Much of the spin is side spin/slice (actually even in "topspin" groundstrokes as John Yandell found), which can be imparted by an equatorial glancing direction, not needing strict low to up brushing. Regardless, that can also be imparted over the contact during the apex region.

 

[toly]

Look at the position of the ball wrt the top white railing. It is constantly falling. First, it was following due to gravity, and it is still falling post impact.

If it is falling, it could not have been hit up with that high of a swing speed, gravity notwithstanding.

If the ball was hit up, we would have seen at least a momentary increase in height. We see only decrease.

The spin in explained by the action over the dwell time. The ball is caught just below the apex by the strings, and the friction during the upward movement of the strings is the one that creates top or side spin. When the ball is released, it comes off with the spin on it, but slightly downwards, as the frame has moved over the apex.

That is how these serves are hit. Not taking the dwell time at the apex into account results in paradoxes like how come the ball has top spin but is moving down after impact, etc.

It is not simple by any means.

Let us assume that the coordinate system associated with the falling ball. Then, from the ball standpoint, the racquet moves upward, thus it must impart topspin.

 

[LeeD]

bhupaes..
I think the statement is wrong. It should be....MOST of the top pros use a significant amount of topspin on their first serves.
Guys going for pure MPH don't add spin to slow down the serve.
Guys often hit fast SECOND serves as their first serve, so those askew the statistics.
And adding spin always increases arc, so the ball goes IN, something MOST, but not all pros do.
I''ve always maintained that Sampras could easily hit into the 150's, but times and clocks have shown he serves around 137 max, with huge amounts of spin. Flattening the ball would account for the dicrepancy.

 

[sureshs]

Let us assume that the coordinate system associated with the falling ball. Then, from the ball standpoint, the racquet moves upward, thus it must impart topspin.

True. That could also explain the topspin without the racket moving up. But it depends on how long the ball has been sliding down the strings due to gravity. It again comes down to the dwell time near the apex.

 

[sureshs]

bhupaes..
I think the statement is wrong. It should be....MOST of the top pros use a significant amount of topspin on their first serves.
Guys going for pure MPH don't add spin to slow down the serve.
Guys often hit fast SECOND serves as their first serve, so those askew the statistics.
And adding spin always increases arc, so the ball goes IN, something MOST, but not all pros do.
I''ve always maintained that Sampras could easily hit into the 150's, but times and clocks have shown he serves around 137 max, with huge amounts of spin. Flattening the ball would account for the dicrepancy.

You are right LeeD:

"Research showed that Pete had about 2,500 rpm on his 1st serve and around 3,000 rpm on 2nd serve."

 

[toly]

Much of the spin is side spin/slice (actually even in "topspin" groundstrokes as John Yandell found), which can be imparted by an equatorial glancing direction, not needing strict low to up brushing. Regardless, that can also be imparted over the contact during the apex region.

IMO noteworthy spin can be done by using very fast wrist ulnar deviation. Rionic hits with bend elbow, so his ulnar deviation is very slow and he cannot create a lot of sidespin.

 

[LeeD]

It's tough to indicate what is positive or what is negative when we use ONE specific player example.
Milos is 6'5", super ape armed like old ColinDibley (6'3" and some change). Colin could reach almost 9' on his fingertips, flat footed.
Milos is taller.
Now how does a strikepoint at around 10 compare to OUR strikepoint heights of maybe lowest 9's?
Different techniques? Certainly a huge difference in angle of ball leaving the racket, as the net is a constant.

 

[sureshs]

What had really amazed me was observing old short men hitting down on the ball and getting the flat serve in (with a very good percentage, honed over the years). These guys would not know top spin even if it crawled up their behinds. That is what led me to look into the (wrong) geometrical proof that has been around all these years claiming that you have to be 9 feet tall or whatever to put the ball into the box if you hit down. It was simply a lack of knowledge of the parabolic shape of projectile motion (exact without air drag, approximate with air drag).

 

[toly]

True. That could also explain the topspin without the racket moving up. But it depends on how long the ball has been sliding down the strings due to gravity. It again comes down to the dwell time near the apex.

IMO the major factor is ball vertical speed. The higher toss the more topspin we can get. Importance of dwell time is not so obvious, at least to me.

 

[sureshs]

IMO the major factor is ball vertical speed. The higher toss the more topspin we can get. Importance of dwell time is not so obvious, at least to me.

I did not mean value of dwell time is important - I said whatever happens, happens within it.

 

[bhupaes]

bhupaes..
I think the statement is wrong. It should be....MOST of the top pros use a significant amount of topspin on their first serves.
Guys going for pure MPH don't add spin to slow down the serve.
Guys often hit fast SECOND serves as their first serve, so those askew the statistics.
And adding spin always increases arc, so the ball goes IN, something MOST, but not all pros do.
I''ve always maintained that Sampras could easily hit into the 150's, but times and clocks have shown he serves around 137 max, with huge amounts of spin. Flattening the ball would account for the dicrepancy.

Agreed. Really tall guys probably could hit down on the ball more and still get good consistency...

 

[Relinquis]

What had really amazed me was observing old short men hitting down on the ball and getting the flat serve in (with a very good percentage, honed over the years). These guys would not know top spin even if it crawled up their behinds. That is what led me to look into the (wrong) geometrical proof that has been around all these years claiming that you have to be 9 feet tall or whatever to put the ball into the box if you hit down. It was simply a lack of knowledge of the parabolic shape of projectile motion (exact without air drag, approximate with air drag).

by parabolic, do you mean "round/curved"? I don't speak physics that well.

 

[sureshs]

by parabolic, do you mean "round/curved"? I don't speak physics that well.

Yes, like the cross section of a satellite dish

 

[Chas Tennis]

High Speed Video Interpretation of Impacts on the Serve

Reference the ball position by the guy in the white T-shirt in the stands behind Rionic. On frame 1 & 2 the ball is falling and lines up with his arm. On frame 3 the ball has considerably moved forward. The ball impact was after the exposure of frame 2 and before the exposure of frame 3, as bhpaes said.

There are some replies in this thread on the angle of the racket face at impact on the serve. Some considerations for our amateur analyses of single camera 2D high speed videos of ball impacts on the serve -

I recently realized some timing issues when looking at 240 fps frames of the ball impact on the serve. This involves the frame before contact, the frame with contact (or closest in time to contact) and the frame after contact. It explains some things seen on many high speed videos in the 200-300 fps range such as usually seeing the ball in contact with the strings.

To illustrate, assume that the frame rate is 240 fps and the shutter speed is very fast such a 1/10,000 sec (motion blur is not significant).

(1/1000 sec (one millisecond) is a suitable unit for time)

1) At 240 fps the camera takes a frame every 4.2 milliseconds.

2) The shutter speed is only 0.1 millisecond. (At 100 MPH an object moves 1760"/second and with a 1/10,000 sec shutter speed the motion blur across the frame would be 0.176" - you can see the racket sharply.)

3) The contact time for the ball, that is, for the ball and strings to deform and then separate - is usually estimated in the 3-5 millisecond time range. Assume it's 4 milliseconds for this discussion.

Then a frame is taken 4.2 milliseconds and the ball is in contact with the strings for about 4 milliseconds. This explains why I almost always see just one frame showing ball impact when using 240 fps.

Earlier in this thread there was some discussion of whether the racket was slightly open or closed at impact. This issue is probably very important as to whether the serve is good or a fault. This issue cannot be answered with high speed video at 240 fps, a higher frame rate is necessary.

Another interesting point is the very high angular rate at which the racket is rotating tip over handle:
For 240 fps nearly always
1) The frame before impact the strings face up - open.
2) The frame with impact is near vertical but because of the slow 240 fps frame rate we don't know at which point during the 4 milliseconds of contact the frame was taken. Needs more analysis since contact last 4 milliseconds and the racket is rotating about 12° in that time. See measurement below.
3) The frame after impact the racket face is closed.

How much are these before-and-after impact angles and what is the rotation rate of the racket?

Raonic serve similar to that in reply #39 video (240 fps) - measure the angles before, during and after impact.

1) Frame before impact: -14°
2) Frame with impact: -2° (note this impact frame appears to be late, the ball is displaced and almost separated from the strings.)
3) Frame after impact: +13°

In the frame time before impact the racket would have traveled about 12° in 1/240 seconds and was still open 2°. The racket appears to travel 15° after impact - inaccuracy in these crude measurements of a 3D moving object with a 2D camera (?) or maybe the racket continued to accelerate. ?

The frame before impact to the frame of impact gives an angular rotation rate for the racket (as viewed by the 2D camera) of

12°/0.0042sec = 2860°/sec

Does this rotation rate result from the wrist flexion during the final approach to the ball when the forearm-racket angle β goes from about 90° to, say, 25° at impact. ?

See Table 2 Wrist Flexion
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2577481/