Re: .Bernoulli and curve balls.

[palmer@sfu.ca (Leigh Palmer)]

> But  my original question was 'does Bernoulli give the wrong direction'.  I
> really can't believe that generations of texts (including many still used)
> REALLY have this wrong.  But my question still holds--for a ball moving
> towards the top of the screen, spinning counter-clockwise as viewed from
> above, which way does it curve?  Bernoulli (at least the presentations I've
> seen) says TO THE LEFT.  To my mind that is IN THE DIRECTION OF THE SPIN.
> IS THAT WRONG?  My sense of most responses is that it is NOT!

Let me apply Bernoulli's principle in a straightforward manner and you
tell me where I go wrong I will concentrate my attention on the surfaces
of the ball toward the right of the screen and the left of the screen.

1. By my reckoning the relative speed of the airstream over the
   right-hand surface is greater than that over the left-hand surface.

2. According to Bernoulli's principle the surface with the higher
   airstream velocity should experienec the lower air pressure.

3. The force which acts on the ball from the right-hand side is less
   than the force which acts from the left-hand side because the air
   pressure on the right-hand side is less.

4. There is a lateral acceleration of the ball to the right because
   there is a net force acting on the ball to the right.

My sole objection to this explanation is that the unbalanced force
seems to be to the left, at least if I am to believe Newton.

Now I want to know what is incorrect in my "explanation" above. Please
don't say I forgot that there is a layer of motionless air in contact
with the ball; I already know that. It is present whenever an airstream
moves over any surface, and that fact is irrelevant to other explanations
based on Bernoulli's principle. I read your (Miller's) explanation; it
raised that irrelevancy so forget it.

> THE 1% THAT GO ON will understand that Bernoulli was a gross
> simplification--THIS STATEMENT DOES NOT SAY THAT the other 99% DON'T see
> massless strings etc as simplifications.  Your response IS NONESENSE--AND
> RUDE!

I'm a rude fellow, but at least I didn't shout.

> Again the majority of physics students don't define direction of spin the
> way PhD physicists do in the midst of an introductory course.  They WOULD
> view a ball with backspin as spinning 'up', overspin as spinning 'down' and
> right and left spin as I've described.

Well, you have the majority of your students in mind. I'm pleased to be
able to report that the majority of my students (by actual count) take
the spin direction to be along the axis and they know the counterclockwise
positive convention, and I think they still remember that I told them it
is only a convention, but I didn't test them on that.

> Part and parcel of teaching introductory courses to non-physics majors is
> to make the subject interesting and relevant to their lives.  It is also
> necessary to draw on their experience and familiarity with phenomenon.
> FLUIDS, for those of us who still deal with this area, offers MANY such
> examples.  Bernoulli offers a bridge to fluid phenomenon using Energy
> Conservation principles along with Mass conservation--continuity
> equation--topics that are usually central to this level course.  The fact
> that almost all the TEXTS do present phenomenon such as curve balls,
> airfoils, and the like with these simplified explanations is reason enough
> to cover them--UNLESS one is telling  students NOT TO READ the books--or is
> so great a lecturer that a book is unnecessary.

I certainly agree that teaching fluids in an introductory course ought
to be considered necessary. I certainly do point out errors in their
books. I don't tell them not to read the books; I simply tell them not
to believe those things that are wrong! I think it is very healthy to
let the students in on the fact that errors in textbooks are not
uncommon. It encourages them to think critically.

Since you didn't bite on my curling problem, let me put in a word for
another common textbook problem together with a related error. Most
textbooks treat heat transfer by conduction and by radiation
quantitatively, but they only mention convective heat transfer in
passing. Convection is the most important of the three in an
environmental context, but it continues to be neglected because it
doesn't admit of analytically elegant solutions. Even "Newton's Law
of Cooling" seems to have disappeared from elementary texts, though
it is surely a physically appealing construct even if it is not as
exact as the other heat transfer laws. If we wish to appeal to the
nonphysicist as a relevant discipline can we continue to ignore the
significant physical phenomena just because they are aesthetically
unappealing? Surely the business of Reynolds numbers and such is not
mathematically difficult. It involves only simple algebra. But it
lacks the elegance of form we see in, for example, radiative heat
transfer, and it lacks the precision of that phenomenon as well. The
fact that it is useful seems to be insufficient justification for
teaching it.

I don't think I've harped on it before in this group, but how many
of you out there are teaching your students that a greenhouse gets
warm because of the greenhouse effect, the same effect that heats
planetary surfaces with atmospheres? That's a lie, of course. It
has been known since the teens that greenhouses are heated by the
supression of cooling by convection. R. W. Wood (who probably knew
that all along) proved it by making a test greenhouse out of rock
salt. Today we use polyethylene sheet for some greenhouses, the
same material that is used for infrared windows in vacuum apparatus.

The greenhouse effect is easy to explain to students, but it is
*wrong*. These same students whom some of you take to be too dull
to handle the truth are often clever enough to leave their car
windows open a little to keep it from getting too hot inside when
it is left in the sun. They understand convection. Is it any wonder
that they begin to see physics as being irrelevant to explaining
the real world?

> Its Friday, I'm getting more annoyed by this thread than educated--so I'm
> 'out of here' for the weekend!

Next time you don't want to hear the right answer, don't ask the
question.

Leigh