Moment of impact in sports, what's really going on?

  • #1
Lactobacillales
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Hello Physics Forums,

I enjoy playing pool, my knowledge of physics is almost nonexistent, so I apologize in advance for my ignorance.


In a nutshell, I would like to understand how the hardness of a pool tip effects energy transfer in that brief moment of impact when a pool cue tip hits the cue ball. A common shot in pool is called a "draw shot" where you hit below the center axis on the cueball, imparting a backward spin, when the cue ball makes contact with the object ball it reverses direction due to the backward spin.
Follow-and-Draw-Blog-300x154.png


Supposedly "a soft tip at slow speed has a longer contact time (about 0.002-0.003 sec), a very hard tip (e.g., phenolic) at fast speed has a shorter contact time (about 0.0008 sec)".(1)

In the pool world, there is a highly regarded gentleman who has his doctorate in mechanical engineering and he states that a soft tip doesn't impart more spin than a hard tip (where I got the above information), although that doesn't match my anecdotal experience as a life long pool player.

Golf a much different sport, with many different variables, it is widely accepted that a softer golf ball stays in contact longer with the golf club, thus imparting more spin, here is study stating as much,

"Gobush (1996a) measured the coefficient of friction between both soft covered three-piece
and hard covered two-piece balls and a club insert that was mounted on a force transducer.
The coefficients of friction were determined from both the ratio of the measured transverse and
normal forces and from calculations using measurements of the velocity components of the
balls before and after impact. For a relatively high angle of incidence of 70˚ and an incoming
tangential speed of 12.8 m s−1, both methods gave a value of approximately 0.38 for the three-
piece ball and 0.16 for the two-piece ball. The rebound spin rates were found to be 66.2 rps
for the three-piece ball and 25.5 rps for the two-piece ball. For a greater incoming tangential
speed, 26.8 m s−1 for the three-piece balls and 25.9 m s−1 for the two-piece balls, the average
coefficient of friction decreased to approximately 0.29 and 0.075 respectively." (2)
"​
At the end of the day by my logic, you have the golf ball in contact with the clubface and the cue tip is in contact with cueball for just a fraction of a second. I don't understand why cue with a soft cue tip wouldn't impact more spin in that millisecond, the way more spin is imparted using a golf club with a softer face or a softer ball?

Can comparisons be made between the two sports?
Thank you for your time!


1. https://billiards.colostate.edu/faq/cue-tip/hardness/
2. http://www.freepatentsonline.com/7762911.html
 

Answers and Replies

  • #2
hutchphd
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Without going into detailed analysis, I think the issue is whether the geometry of the event is changed by the "hardness" in question. Even the softer cue stick deforms very little during the slower but longer interaction whereas the the deformation of the golf ball is likely considerably changed.
I think an even better example is afforded by tennis where relatively small changes in the tension of the strings leads to remarkable changes in racket "feel" and response.
 
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  • #3
Lactobacillales
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Without going into detailed analysis, I think the issue is whether the geometry of the event is changed by the "hardness" in question. Even the softer cue stick deforms very little during the slower but longer interaction whereas the the deformation of the golf ball is likely considerably changed.
I think an even better example is afforded by tennis where relatively small changes in the tension of the strings leads to remarkable changes in racket "feel" and response.
Thank you for the response Hutch, I decided to look at tennis after you mentioned it, I found an interesting article which was well supported, and the conclusion was "Changing racquet tension does not affect spin, but it does affect string movement, dwell time, and ball contact distance. These latter parameters all can affect the ball trajectory as well as the player’s feel of the impact"(3).

I am confused as to why you would recommend tennis over golf in terms as a better example, wouldn't the deformation of a tennis ball be greater than that of either a pool ball or golf ball?

I assume because a golf club face is so much harder than either a tennis racket or cue tip, although I did find this patent where the clubface being modified (as opposed to different golf ball materials) increased spin, "Kitaichi invented a golf club head with elastic intermediate applied to the clubface. When launching the ball, the elastic deformation of the elastic intermediate impart excessive backspin to a golf ball due to longer contact time" (4).

I am also open to reading and trying to educate myself, I am just not sure where to start, if you might point me in the direction of some terms I should search so as to better articulate myself.

Thanks again!


3. http://www.tennisindustrymag.com/articles/2005/01/does_higher_string_tension_giv.html
4. https://web.wpi.edu/Pubs/E-project/...113722/unrestricted/Golf_MQP_Final_Report.pdf
 
  • #4
hutchphd
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I am confused as to why you would recommend tennis over golf in terms as a better example, wouldn't the deformation of a tennis ball be greater than that of either a pool ball or golf ball?
My supposition was that the geometry of the tennis event is more sensitive to the racket tension because both the ball and the racket deform. I know there are good tennis players who believe the amount of spin that can be imparted to the ball is very sensitive to racket tension...I have never reached that level of control!

In terms of actual study I can only direct you to google and the terms "topspin" "sidespin" and perhaps racket (racquet?) tension. There are some sports physics books but I don't know them well enough to recommend. Somebody else likely will. Have fun.
 
  • #5
jbriggs444
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Possibly a relevant analogy would be to attempts to spin a raw egg.

If you attempt to spin an egg with a rapid motion, you won't get very far. The soft insides will not come up to speed rapidly. If you spin it up slowly it will spin well. [The effect is more easily seen when trying to stop a spinning egg. A brief stop won't do the job]
 

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