Quote:
Originally Posted by 10isfreak
To give credence to your estimation, let's validate the approximation. Most pros hit at an angle less than 20 degrees and usually higher than 10 degrees (in terms of slope). We could deal with this using the tangent and a 100 units long side in a rectangular triangle.
Tan(20) = x/100 which leads us to 100 tan(20) = x = 36,4
Tan(10) = x/100 leads us to x =17,6
So, between 17,6 percent and 36,4 percent of the vector is oriented upward. With 35 m/s, it's 6,17 to 12,74 mm of upward movement.
We may have a good case to think that, indeed, the ball might move... An alternative possibility is that the ball moves along with the racket during this dwell time; yet an other possibility is that of a combination of sliding ball and moving ball. If we believe SpeedMaster's analysis (that much of spin is generated through the racket tilt and a contact bellow center  and by bellow, I mean closer to the ground, not the throat), only a fraction of this sliding effect would contribute to spin and this fraction should be less 50%, to say the very least.
A good way of finding if someone is educated or not is asking this person if things like this are complicated or not to understand. If he answers the later, he's either a genius who didn't reveal his conclusions or he sees the tip of the iceberg.

I have seen that 18 degree number too but I think that is mostly on balls hit on chest Level. on balls below waist the upward swing angle is probably more than 30 degrees.
when fed hits a low ball he will swing up much more. the upward angle is not only to create spin but also trajectory. to hit a ball at the knees with 3 feet net clearance you Need quite a bit of lift.
here is a nadal FH on a low ball with more than 45 degrees upward swing
http://www.youtube.com/watch?v=NSr6dfxhMUw
and here are higher balls with a flatter swing (more like 25 degree I estimate)
http://www.youtube.com/watch?v=NSr6dfxhMUw
and on chest high balls he will swing probably even flatter. swing trajectory has a Lot to do with strike height.