[From Bruce Abbott (2009.05.14.0835 EDT)]
Why do curve balls seem to “break” suddenly as
they near the batter? Check out http://www.aip.org/isns/reports/2009/051309visualillusion.html
for a demo and an explanation. It’s a fascinating illusion, and you can
play with the parameters in the demo!
Bruce A.
[From Bill Powers (2009.05.14.0650 MDT)]
Bruce Abbott (2009.05.14.0835 EDT)]
Why do curve balls seem to �break� suddenly as they near the batter?
Check out
http://www.aip.org/isns/reports/2009/051309visualillusion.html for a
demo and an explanation. It�s a fascinating illusion, and you can play
with the parameters in the demo!
AAAArgh. That is not how a ball looks to a batter, it’s how it looks to a
seagull flying overhead. Furthermore, a ball pitched at 60 miles per
hour, a slow velocity appropriate for a breaking ball, covers the 60 feet
to home place in 60/88 = 0.68 seconds, not the 2 sec or so the animation
takes. And finally, the background against which the ball is seen by the
batter is not a featureless gray but is highly textured, so autokinetic
effects (which is what I think is going on here) do not occur.
A breaking ball appears to break because it is (a) accelerating normal to
the line of travel, and (b) getting closer to the batter. The vertical
acceleration (lateral for a curve ball) is due to the spin which produces
a force acting sideways or vertically on the ball, so that component of
velocity increases with time and is fastest near home plate. The
departure from the normal trajectory is further exaggerated by the
approach of the ball so a given sideward velocity appears as an
increasing angular velocity relative to the direction of gaze. A
downward-breaking ball appears to break the most because gravity adds to
the downward force due to the top spin for that kind of break, thus
making the downward velocity accelerate even more.
Note that the Bernoulli effect does not explain the curvature of the
ball’s path. The Bernoulli effect says that air pressure is reduced when
air flows past, and parallel to, a surface. The ball curves the wrong way
for that to be the explanation. The real explanation I have heard is that
the boundary layer of air is dragged around with the spin so air pressure
is increased on the side of the ball spinning toward the direction of
travel. A ball with top spin therefore accelerates downward faster than
it would under just gravitational force. Anyone who plays ping-pong well
knows this.
People have been trying to prove that curve-balls are an illusion since
pitchers discovered them. The example you cite is way off the mark. Ball
Four!
Best,
Bill P.
[From Rick Marken (2009.05.14.0720)]
Bill Powers (2009.05.14.0650 MDT)–
Bruce Abbott (2009.05.14.0835 EDT)]
Why do curve balls seem to break suddenly as they near the batter?
Check out
http://www.aip.org/isns/reports/2009/051309visualillusion.html for a
demo and an explanation. Its a fascinating illusion, and you can play
with the parameters in the demo!
AAAArgh. That is not how a ball looks to a batter
Perhaps not. But it is a super cool illusion!
Best
Rick
–
Richard S. Marken PhD
rsmarken@gmail.com