Originally Posted by dominikk1985
Regarding the sliding on the strings:
yes the contact time is short but the Speed is also high.
a 100+ mph Forehand with a lot of spin has at least 80 mph RHS.
that is 35 m/s. asuming that the contact time is 3/1000 of a second the racket moves 35/1000*3=0.105m or about 4 inches.
of course not all of this is up since the swing is also Forward but it is not unreasonable that about a third of it (33% upswing angle) is upward which would mean that the racket moves about 1.5 inches up during contact time.
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.