THE BOOK--Playing The Percentages In Baseball

I did not get a chance to read this carefully.  However, I was immediatly struck by one commment:  that the Magnus force can make a ball move faster.  That is simply not true.  The conventional Magnus force is perpendicular to the velocity vector, so it can only change the *direction* of the velocity and not the *magnitude*.  Of course, it can make a given component of the velocity larger, but only by making another component smaller.  Perhaps I am misunderstanding what the author is saying, due to my very quick scan of the article.

Alan, I had to think about that comment for a little bit, and maybe I didn’t quite get what was being said, but this is how I took it:

The Magnus force can change the path that the ball takes.  The Magnus force flattens/straightens the path of a fastball.  A straighter path means that the ball gets to its destination faster. 

In other words, if you released an 80 mph fastball and an 80 mph curveball from the same release point and aimed them to arrive at the same final point, the 80 mph fastball would get there quicker because it would take a shorter path.

Now, I have to think about whether that is true, but that is what I understood was being suggested by the comment.

After thinking about this a bit, I want to qualify my earlier post.  While it is definitely true that the speed of the ball is unaffected by the Magnus force, perhaps that is not the main issue.  What matters is how long it takes the ball to cross the front plane of home plate, which depends on the x-component of the velocity (I am using the notation in the article and Tango’s diagream and not the usual pitchf/x coordinates).  Since the ball must fall on its way to home plate, then the Magnus force will have a x component toward from home plate if thrown with backspin (as in Tango’s diagram) and toward the pitcher if thrown with topspin.  Thus, a ball thrown with backspin will have a slightly higher x component of velocity and will get to home plate quicker.  The opposite is true for topspin.

One additional thought:  In Tango’s diagram, the y-component of the Magnus force is said to make the ball appear faster.  The essential physics there is that the upward Magnus force results in less drop on the ball, which is exactly the same effect that one would have with a ball thrown faster but with less backspin.  In that sense, an extra “hop” on a fastball due to additional backspin looks to the batter like a higher speed.  Not that the ball gets to home plate quicker but that the ball does not drop as much.

Mike/2, Alan/3
Yes, that is exactly what I was trying to say. The Magnus Force can of course not increase the ball velocity itself, but it does add to the VMG (Velocity Made Good, to use a sailing expression) because it moves the ball towards a shorter path toward a strike zone plane.

Also, sorry for making it confusing with the non-conventional usage of X and Y.