Hitting up vs. Brushing up on the serve

[toly]

Originally Posted by sureshs

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]

Originally Posted by toly

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]

Originally Posted by 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.

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

[Relinquis]

Originally Posted by 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).

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

[sureshs]

Originally Posted by Relinquis

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]

Originally Posted by Chas Tennis

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/