Originally Posted by JohnYandell
Just so others may note--I asked a simple question about an arc that wasn't answered.
It is just so fundamental that a point (in this case the center of the eventual impact area on the strings) that moves in a 3D arc must have forward, up and across components of velocity! As per video analysis, the forward velocity was about twice that of the upward one, which explains (even including gravity) why the 3D volume of tennis is extended in the horizontal plane, unlike say, badminton (except for lobs) (I wanted to raise this in the other thread when Oscar said tennis is a vertical sport not a horizontal one, but decided not to in the presence of all the name-calling. Badminton is more of a vertical game with the bird often being flighted high).
There is movement of the ball in the forward direction, and this is the simple meaning of extension. Forward need not be "linear" - it can be angular and cross-court - just into the court is the meaning. And any physical system which provides a force to move in a certain direction must move in that direction at least some moments before and after impact - inertia will not allow abrupt yanks, pulls, etc. There is of course also an upward velocity which produces the top spin.
This is pretty simple and I don't understand why people split all these up into pieces and claim that no one understands only one of them.