More Spin - High Speed or High Accelleration?

Alois,

Thanks for the answer.  I thought it was good suggestion, so i went to the site www.askthephysicist.com - here's my question and his answer:

QUESTION: 
i just took up Table Tennis again after a long layoff and as you may know friction (imparting spin to the ball) is a large part of the game. I was surprised to read in one book that the amount of spin imparted to the ball from the paddle during the stroke was not a function of the SPEED at which the paddle "brushes" (almost tangentially) the ball, but of the ACCELLERATION while the paddle is in contact with the ball. They suggested approaching the ball slowly and accelerating through impact . I wrote to an "Ask the Coach" forum and the coach said he thought it was just a function of speed but asked if anybody knew the physics of the problem. I think i understand the difference between static and dynamic friction, but since the paddle isn't starting from rest, just going a constant high speed in one case vs. slow to fast in the other, i didn't think this would apply.

ANSWER:
Here is all I can think of. Because of inertia and limited frictional force, hitting the ball with a surface moving very rapidly may result mainly in slipping of the two surfaces which would not be so good for imparting spin. So, starting slow to allow the paddle to "grasp" the ball more effectively might result in more transfer of spin.

 So since with all your experience you can't tell a difference and the Physicist seems to be grasping a little - I'm going for max speed - that seems much easier for me to concentrate on, anyway.  Thanks!!

Hi Wayne,

Thanks for going to all that trouble.  Sounds like we can just stick to working on speed.  If anyone else has more knowledge, please let us know. 

I designed my backhand push around the notion that higher acceleration would produce higher spin.  My thought being that starting slow and speeding up while the ball was in contact with the paddle would keep it in the rubber a bit longer and give more of a flinging action.  It was rather successful in the short run -- I had a formidable push.  The problems were:

• relying on a deep push kept me from developing a good topspin response
• the extra strain on my elbow contributed to tennis elbow
• the margin for error is smaller than when using a fast, smooth motion.

As it so happens, I do have a physics background (that was my uni major which was yonks ago and I've forgotten everything).

Basically, following on the answer already given, friction is what will get the ball spinning.  Now there's two broad types of friction, static and kinetic.  If two objects (like a racquet and a ball) come into contact already traveling at speed relative to each other, kinetic friction comes into play.  If two objects are stationary to each other, and in contact, and THEN start to move relative to another, static friction comes into play.  Basic rule is static friction is higher than kinetic friction.  You can see this if you do a spinout in a car.  Originally the wheel and the surface are stationary to one another, but as soon as you overcome the static friction, kinetic friction takes over and then there is very little grip.

You could argue, if you had a very slippery racquet, that likewise it would be best to start contact with the ball in a stationary fashion and then gradually accelerate to ensure you never break static friction.  However, given the short time of contact between racquet and ball, this would be pretty useless.  That's why we pay the big bucks to get super duper tacky, sticky, high friction rubbers. 

Key thing is to generate the fastest possible racquet head speed in the distance/time you have available in a stroke.  My guess is you need every inch of that distance/time to accelerate as quickly as possible from start to finish...you don't have time to deliberately go slow at the beginning in order to accelerate later on.

I think the contact time is very short if you are looping the ball so you should go for max speed.  However, in order to achieve max speed at contact, you will need to go from slow to fast (accelerate). If you start fast and try to keep fast speed throughout the swing (like I did before), you are going to lose both speed and control at the time of contact because your arm can not maintain a high speed for the whole swing, neither can it control the swing well at a constant high speed (it gets tired before you hit the ball). As a result, the spin on the ball will be less or you have a chance to miss the ball. Actually, you don't need a lot of power to generate spin and the rule is never sacrifice control for power. So I would suggest accelerate and achieve max speed at contact.

I think the quote in the original question is in McAfee's book.  Actually he said that in connection with ball speed, not spin.

To increase ball speed, more energy should be transferred to the ball.  The energy E is equal to the work done on the ball, which is equal to the force applied times the distance over which it is applied (E=W=F*d).  Force is mass times acceleration of the ball (F=m*a).  So to get high ball speed the ball must be accelerated. On the other hand spin depends only on how fast the contact is and how fine. These are apparently conflicting requirements.