SA,
If I knew how to do multiple quotes within a post I would. Please bear with me. Your comments are black mine are red for clarity purposes. As you can tell, I am all about clarity.
In response to #59.
"The Magnus Effect (due the topspin & sidespin) tells us why the ball flight is curved but it does not tell us why the ball bounces the way it does. The orientation of the spin axis to the direction of the ball flight is important because it gives us clues as to what types of spins are involved. It is generally accepted that the spin axis is not perpendicular to the flight of the ball for a Twist serve while it (pretty much) is perpendicular to the flight path for other types of serves."
I agree with the Magnus effect. It tells us why the ball curves. I agree it does not tell us why the ball bounces to the right..I also agree that the spin axis is not perpendicular with a twist.
"I also maintain that bounce characteristic is due to the ground interaction of the type(s) of spin involved, and not a function of the trajectory of the ball (or the change in spin axis orientation with respect to the lines on the court)."
Here is a main focal point. I agree TOTALLY with your ground interaction statement. I TOTALLY disagree with you saying "not a function of trajectory". Why? Because as the trajectory of flight changes (flight bending down and to the left) the axis of spin changes in relation to the court (surroundings-up, down, left, right). Take for example a football. Its z axis is from back to front right? It spins on this axis when thrown right? When the ball is thrown and it is flying upward, the z axis of the ball is pointing upward. When it falls the z axis is pointing downward.
Now imagine if it were possible to throw that football and make it fly like a twist serve on a tennis court. It goes up, some out to the right, over and down to the left. Somewhat like a corkscrew. It bends in flight. It changes directions. The z axis of the ball changes in relation to its surroundings, ie the court. The balls surrounding would include the court surface, lines, nets, fences etc. Following this flight pattern or trajectory, when the football hits the court, the z axis is no longer pointing in the same direction as it was when it left your hand. Why? Because the ball is flying downward and to the left at impact with the court, the ball changed direction thus changing it’s a axis in relation to it surroundings... This example of a football is totally imaginary I understand. I am just trying to explain or illustrate how the relationship of the axis of spins on a ball change due to its flight trajectory, in my own way of course. And I have no problem with it being convoluted. I don’t know anything about x,y,z axis as they are taught in science classes. However, I am correct about this idea. And this idea of axis of spin changing fully explains the sideways kick for a tennis ball twist serve.
"I had previously stated that the spin axis orientation is pretty much constant with respect to the flight of the ball itself. I also said that I believed that the orientation is fairly constant with respect to the court surface. However, I did not say that the spin axis orientation was constant with respect to the net or the lines on the court. I agree with you that this latter relationship changes as the ball curves in flight."
It is not true that the axis of spin remains constant in respect to the flight of the ball. It would be true if its flight were straight, but it isn’t. It is true that the axis of spin remains constant in respect to the ball itself. See above example of football...And you cannot differentiate between the court surface and its lines or the net. They are all objects that are fixed and do not move. If there is a relationship change between one, then there is a relationship change between the others as well.
"I am still not understanding why you talk about the horizontal axis without also talking about the vertical axis. Doesn't your angled axis already take both topspin and sidespin into account? Why would you talk about another horizontal axis spin if you say that the ball has an angled axis spin."
No my angled axis does not take any other spin into account. Each are separate. There are two distinct spins, topspin& angled side spin. Look at the link you provided. On that web page the guy illustrates the axis of spin and uses angled sidespin as one of them and ALSO illustrates the twist angled topspin as well. I realize that some people do not believe the two spin idea. And it is difficult for some to grasp. But take in consideration a Nascar race car that has a wreck. We see them fly through the air twisting in multiple directions all at the same time. The car is tumbling end over end and twisting at the same time. This tumbling twisting mass is two directions, you can see this with your own eyes. This tumbling twisting mass can rotate a third direction as well. A flying object can rotate in three directions at the same time. This does not mean it always will, or it absolutely has to. I am simply saying it can and will. And the car example is a good one because you can see this happen. But it is important to realize that if, I say if, the car were to rotate say 2500 rpm, this rotation would be so fast it would be difficult to ascertain direction of spins with the naked eye. The car has a front end and back end and corners like left front and back rear. These structural characteristics make it easy to SEE the rotating motion(s). With a ball or sphere it is difficult to see these things. Herein lies our difficulty, along with a lack of technical writing skills on my behalf.
I am speaking of a horizontal axis because it is the result of the upward brush of the racket. I am speaking of the angled spin axis because IT is the result of the 8 to 2 brush. The racket is moving in two directions, both up and outward. Thus, it imparts two primary spins on the ball. One with a horizontal axis and the other with an angled axis. I am claiming the ball is spinning in two different directions which are not 90 degrees apart from one another. No, they are more closely aligned. Draw a vertical line on a tennis ball for topspin. Draw a line on it that is from 8 to 2. Intersect each of these lines with a pencil. Yeah, cut holes in the ball and stick pencils through them. Use this tennis ball as I described in my last post. You will see that when the ball changes direction, (trajectory) down and to the left, the direction of the two spins on the ball will have changed in respect to the court. It (the ball) flies (travels) in a cork screw manner. That is, out, up&over and then down to the left. When the ball hits the court is it traveling in a different direction than it was when it was hit. The ball rotates a little bit due to the curvature of its flight. It is clockwise rotation. And it is minimal. But it is enough so that the primary two spins on the ball are aligned to the right. Thus the sideways kick.
The argument between a topspin slice and a twist is simple. The topspin slice only has one spin on it. Read about the ball machine example. Visualize the ball machine laying at a 45 degree angle. Visualize how the ball will fly. You can do a topspin slice this way. But you cannot do a twist.
I have to go. I hope someone can visualize this. It is really simple. I hope someone will take the time to do the pencil and tennis ball example. It works. I did it last night at home. And it fully illustrates how the two spins get "twisted" toward the side fence.
