🚀 go-pugleaf

Publishers sometimes put out books just for teachers. One such, I admit
to buying:

Jewett, John W. _Physics Begins with an M, Mysteries, Magic, and Myth_,
Longwood (Allyn and Bacon), 1994.

Then I got so busy I didn't read it right away to evaluate it and
possibly return it for a refund. It cost about $40 paperback.

It isn't *all* bad, and the idea behind it is good. It cites common
misconceptions (myths) about the real world, related to physics, and
attempts to give correct explanations. It also cites some things which
*seem* like magic, but are merely based on physics. So far, so good.

And many of the explanations are quite well done, particularly those
where *references* are cited. In those cases, the author obviously did
his homework.

But then there are cases where the author seemed to be 'winging it', and
in these cases no references are given. Too bad, for the explanations are
sometimes incomplete or wrong. One in particular, on p. 56, is "Why are
there tides?" The 'myth' he intends to correct is:

        In desribing the tides, some books explain the bulbe of water on the
        side of the Earth away from the Moon by claiming that the Moon
        pulls the Earth away from the water.

His explanation is just as bad. In it he says, in part:

        ...Another bulge appears on the other side of the Earth, due to
        inertia of the water and the centripetal acceleration of the Earth
        toward the moon as they both revolve around their common center of
        mass.

This reads like a watered-down version of the seductively wrong
explanation in _Physics for Scientists and Engineers, by Fishbane,
Gasiorowicz and Thornton (Prentice-Hall, 1993), p. 366. To make it worse,
this short discussion loftily and authoritatively refers to Galileo (as
having a wrong explanation) and Newton (who got it right).

Centripetal effects are important and should not be neglected. Still, if
we were to somehow freeze the earth and the moon in their motion, and nail
them down to the fabric of space-time :-) tidal bulges on both sides of
the earth would still be present. I.e., they would be there even if there
were no rotation about the center of mass and no centripetal effects
present at all. Take away the moon and they would *not* be there.

The rotation produces an equatorial bulge, which is a different beast
from the tidal bulge. In the 'frozen' example, the equatorial bulge would be
absent but a tidal bulge would still be present. Any books which talk
about the tidal bulge as being *due to* rotation is off the mark.

And what's this 'due to the inertia of the water'? That explains a *lot*!
I tell my students that the words: 'because', 'due to', 'truth', and 'prove'
should never pass from their lips (or pen) in a physics context. They are
cop-outs for those who are unwilling to sweat the details and forge the
logical connections.

This is another example of what happens when one tries to present a
subject which has a rather subtle and difficult explanation (and in three
dimensions, yet, with motion) and then boils it down to a simple,
conceptual explanation. The concepts come out wrong.

A far better discussion of tidal effects can be found in Newton's
_Principia_, or, for a more recent discussion, with nice computer-generated
pictures see:

Quincey, Paul. "Why We Are Unmoved As Oceans Ebb and Flow?" The Skeptical
Inquirer, v. 18, no. 5 (Fall, 1994), p. 509-515. Paul Quincey is a
physicist at the National Physical Laboratory, Teddington, Middlesex.

Some of you may be aware of good explanations of tides in elementary
textbooks, but I haven't found any.

        -- Donald