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Old 01-31-2013, 05:01 PM   #48
Bionic Poster
Join Date: Oct 2005
Posts: 34,434

Excerpts from my posts on another thread:

That is because most of the energy is in the forward direction with the up and across component coming from forces below the center of mass, and friction with the strings, which create the spin.

One of them is like flicking a vertical wheel on the lower side with an upwards and forwards force. As long as the line of force does not pass through the center of mass (axle) but above it, the wheel will rotate. Another way is to grip the wheel and turn it. These two are analogous to the strike on the ball followed by friction during the dwell time, and both require the across and up component.

If the direction of the force (and hence the swing just prior to it) was merely normal to the ball, it would just go straight. If the across and up direction passed through the center of mass, the ball would also be launched across and up. So it is a combination of the force in the forward direction which also acts up and across in a glancing way, which makes the combination of pace and spin possible.

Continuing on this topic, there seems to be a lack of appreciation of the dwell time and string friction, which is the basis for all the different varieties of strings and pro preferences etc.

Let us take a billiard ball and a cue. How would you impart some rotation to the ball? By hitting it off center so that the line of force does not pass through the center. You won't get much rotation. How would you increase it? You would have to make the contact more grazing or tangential. That is the only way.

In tennis, the ball and strings deform much more, and there is plenty of friction in the dwell time. This gives an opportunity to be less grazing, but still produce spin by an off center hit. The single arc can create both pace and spin without the need for the arc to be very steep. That is the kind of arc used most frequently.

Table tennis is somewhat in between.
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