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01-31-2013, 11:54 AM   #42
rkelley
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Join Date: Sep 2010
Posts: 2,293

Quote:
 Originally Posted by 10isfreak That's not physics actually. What you depict here is a major misunderstanding.
Let's see.

Quote:
 Originally Posted by 10isfreak Spin is the edge of the ball moving relative to its center.
Agreed.

Quote:
 Originally Posted by 10isfreak Let's simplify the problem for you to grasp it.
Was that comment really necessary?

Quote:
 Originally Posted by 10isfreak Let's say the ball is only two-dimensional (it's a circle). If you apply a vector is directly forward-directed to the upper edge, what happens? The edge moves forward, which is the same as saying that the edge which is bellow moves backward . . .
Agreed. But how do you apply a (force) vector to the upper edge of the ball that has a direction that is "forward-directed?" In the case you putting forth the force vector is not in line with the center of the ball, correct? The ball's center of mass is at its geometric center, so the vector you're describing creates a moment around the center of the ball that creates spin, correct?

But a tennis racquet is a plane that interacts with the ball. The point where that plane contacts the ball must always be normal to the surface of the ball. The reaction force from that contact has to go through the ball's center of mass. So using a plane to apply the force, how do I create the moment about the ball's center of mass?

Quote:
 Originally Posted by 10isfreak You do not need to have a vertical force to generate top spin because top spin is not directed upward.
To be specific, you need a vertical velocity component of the racquet's swing path to generate topspin. Force alone, at least on the racquet, won't do it. You can put all the force you want on the racquet. If there hasn't been enough time for that force to sufficiently accelerate the racquet upward to create that significant vertical velocity component you're not going to get much topspin.

Quote:
 Originally Posted by 10isfreak Our real possibilities are hitting the upper edge or bushing the back of the ball.
So how can I hit the upper edge of the ball with a plane (the racquet) without just directing the ball down? Brushing the back of the ball is exactly what I'm talking about.

Quote:
 Originally Posted by 10isfreak Unless you swing at more 45 degrees from an horizontal plane prior contact, most of your swing is directed forward... With a greater amount of force directed forward, it seems logical to assume that accelerating the upper edge is a better strategy.
Here's a couple of videos of Fed and Djokovic. They actually are swinging up at around 45°.