Contact at 10,000 Frames Per Second

[johnyandell]

Let's discuss Paul Hamori's new article, "Contact at 10,000 Frames Per Second"

[jimlosaltos]

Fascinating research and great videos. I'll have to check out Paul's book.

The part about balls not sliding on the racket face is surprising. I wonder if the results would be different if the racket face is moving rapidly upwards, with a large velocity vector perpendicular to the ball's flight path. Also, does the attitude of the racket face matter?

It's hard to get still photos of the possible string action from actual play. Not only would it have to be at the point of impact, but during the fraction of those milliseconds when the ball "might" have slid. I've taken a number of photos with balls on the racket strings, including a few of Fed. None show the strings sliding.

But I found one old Fed photo that clearly shows how stretched the strings are fitting the old Cross & Lindsey description. Doesn't prove anything, but it is a data point. Sorry, but I don't know the source. I believe it is from the Australian Open, perhaps 2011. Please post if you know the photog/ owner/ credit/

I cropped the larger image, then pulled out and enlarged the racket face so you can see how the strings are stretched downward. The racket face is closed. If the date is right, these are probably gut strings. If later, they are gut mains with lux crosses. Either way it is gut doing the sliding and stretching not copoly.

filedata/fetch?id=95238&d=1630779167&type=thumb

[jimlosaltos]

This photo of mine is more for your amusement than for studying the ball.

I used to have a large print of this -- without the enlarged section -- on my wall and would ask visitors to find the ball in the picture. No one did.

The ball is so compressed, you can barely see the ball peeping out from around the side, edge of the racket. In the enlarged circle, you can also see the strings cutting into the ball. Ball is compressed to, I'm guessing 1.5 in thick?

This is of Johanna Konta serving at San Jose in 2018. She can serve in the high teens.

filedata/fetch?id=95240&d=1630781511&type=thumb

[johnyandell]

It doesn't matter. No sliding only deep pocketing. You can go into the HS Archives and scroll down to Ball Racket intereactions.

[jimlosaltos]

It doesn't matter. No sliding only deep pocketing. You can go into the HS Archives and scroll down to Ball Racket intereactions.

These are great. Hadn't run into them before. Thanks.

https://www.tennisplayer.net/members/high_speed_archive/phantom/Ball_On_String_HD_HS/Forehand/?stroke=Roger%20Federer%20FH%20Center%20ArmAction%20CC%20Front1%2010000fps.mp4

[doctorhl]

I recall on occasion, visualizing a deformed ball at about 2 feet after ball contact on a serve. This old style, American twist kick serve off a wooden racket, was deep pocketed and exited slow enough to see with the naked eye. Whenever I observed this flattened ball, I knew the ball was going to kick extra high and hard to my left. There seemed to be more string separation with gut in those days, but I feel like ball contact was more about pocketing, even then. Perhaps someone can capture pocketing with a wooden racket.

[johnyandell]

doctor,
There is an image of that in the article.

[paulhamori]

Dr Hamori here with comments: I sent John a series with my old Jack Kramer autograph strung with VS gut. I thought I would see significantly longer contact time with the wood racquet but it actually turned out to be about the same as with composite rackets. Gut did have a longer contact than poly. About 4.5 ms as opposed to about 4.0 ms. That is significantly longer contact time for sure. My photographs did not show any more spreading of gut than poly. I also sent John a shot with the Wilson Prostaff 85 which is strung with VS as well. What is notable about the picture which is pre-contact, is that the gut strings are roughed up, and spread apart, and have not snapped back. In the old days the players were constantly readjusting their strings between points, you do not see that at all these days with the poly or hybrid stringing. On the posted picture of Federer–great shot, it almost looks like he is playing 100% gut in the racquet(I am aware that he does use hybrid stringing currently, I am not sure what year he started with that–this picture is 2011) and it looks very similar to the picture I sent to John where the mainstrings just simply have not snapped back. I think Federer is catching the ball below the center of the racquet on that shot and it's pocketing down there. However, cannot be totally sure, it is possible that it spread that string and then slid down to where it is now on the racquet. But in all the shots I took I saw absolutely no sliding, as John has pointed out multiple times, only pocketing. Most the shots I took the ball would squash down to about 60% of full height, but the harder one can hit the more compression we would expect and certainly Joanna Konta has a lot more power than I do. I appreciate the comments and interest in the subject, Dr. H

[jimlosaltos]

Dr Hamori here with comments: I sent John a series with my old Jack Kramer autograph strung with VS gut. I thought I would see significantly longer contact time with the wood racquet but it actually turned out to be about the same as with composite rackets. Gut did have a longer contact than poly. About 4.5 ms as opposed to about 4.0 ms. That is significantly longer contact time for sure. My photographs did not show any more spreading of gut than poly. I also sent John a shot with the Wilson Prostaff 85 which is strung with VS as well. What is notable about the picture which is pre-contact, is that the gut strings are roughed up, and spread apart, and have not snapped back. In the old days the players were constantly readjusting their strings between points, you do not see that at all these days with the poly or hybrid stringing. On the posted picture of Federer–great shot, it almost looks like he is playing 100% gut in the racquet(I am aware that he does use hybrid stringing currently, I am not sure what year he started with that–this picture is 2011) and it looks very similar to the picture I sent to John where the mainstrings just simply have not snapped back. I think Federer is catching the ball below the center of the racquet on that shot and it's pocketing down there. However, cannot be totally sure, it is possible that it spread that string and then slid down to where it is now on the racquet. But in all the shots I took I saw absolutely no sliding, as John has pointed out multiple times, only pocketing. Most the shots I took the ball would squash down to about 60% of full height, but the harder one can hit the more compression we would expect and certainly Joanna Konta has a lot more power than I do. I appreciate the comments and interest in the subject, Dr. H

Just saw this. Sorry to be so late. But thank you for the explanations. It will take me a while to digest. And, yes, Konta hits harder than me as well. If it's any consolation we're not alone.

Enjoyed your article. Best / jim