Re: .Bernoulli and curve balls.

[palmer@sfu.ca (Leigh Palmer)]

> First:  The main audience for Bernoulli descriptions of curve balls,
> airfoils, etc. are those 99% who are in an Intro course and won't be going
> on.  Bernoulli is accessible to these students as a 'reasonably good'
> description whereas laminar flow, vortices, Reynolds numbers, and the like
> are not as accessible to them.

But the "explanation" is fraudulent. What is the value of perpetuating
an explanation when it has that character? I think it would be a good
thing to let students know that physicists have integrity, and that
this particular arcanum, like general relativity, is accessible only
to those who have more knowledge of physics. If the explanation had
any socially redeeming quality other than that it unasks the question
I might go for it, but as it stands it looks like a *counterexample*
to the "Bernoulli effect", since one gets the incorrect direction for
the lateral force with straightforward application of the principle.

> For the 1% that goes one, they will recognize later that Bernoulli was a
> gross simplification of the 'real' situation--just as they eventually learn
> that massless strings, frictionless pulleys, air resistance free movement
> and the like are all simplifications to the real world.

Nonsense. 99% of my students know that those are idealizations, not 1%.
That knowledge is not arcane.

> I take the 'common sense' meaning of the direction of spin.  Look at the
> ball from above (for left/right curvature).  Look at the leading edge of
> the ball along its direction of motion.  In what direction is that leading
> edge turning?   If the ball is moving up the screen then counter-clockwise
> rotation is a 'leftward' rotation and the ball should curve to the left.
> Clockwise rotation the opposite.

One could use this common sense direction to predict the effect when
a spinning tire flying through the air hits a wall. It will be
deflected in the common sense direction, right? We can think of the
baseball as having traction on the air in front of it like a tire
has traction on the wall.

There is a problem for the "explanation" I have given above. It
"works" for curve balls, but it does not work for tires!

We already have a perfectly good way of specifying the direction of
spin. It is specified with respect to an axis, and there is no
"common sense" direction ascribable to any direction perpendicular
to that axis. In fact the axial direction of the spin is only
ascribable by convention; but the axis is defined in both technical
and "common sense" terms.

When you've all figured out how to explain all natural phenomena in
terms of high school physics, I would like some help on a problem
I've been wondering about for some time. Why does a spinning
curling stone curl the way it does? Every simple explanation I can
think of has it curling the opposite way. I know it curves in the
"common sense" direction, but I'm unsatisfied by that explanation.