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Re: [Phys-L] this stuff is easy ... or not

What is the difference between an x axis, x plane, and an x contour?

...::. Sent from a touchscreen.::...
Paul Lulai

----- Reply message -----
From: "John Denker" <>
Date: Sat, Mar 16, 2013 7:28 pm
Subject: [Phys-L] this stuff is easy ... or not
To: "" <>

In the context of

On 03/16/2013 05:58 AM, Brian Blais wrote:
Sometimes I think the guide assumes something is simpler (for the
reader) than it really is.

That's unintentional.

Given the choice, I would prefer to err on the side of exaggerating
how easy stuff is, rather than exaggerating how hard it is ... but
really I'd rather not exaggerate either way. IMHO the motto should
be: "This stuff is easy if you know how; let me show you the trick."

Of course if you say that but don't sufficiently explain the trick,
that's a problem, but that's usually a fixable problem. I would
much rather face that problem ... as opposed to saying "this stuff
is hard" and giving up without even trying to explain it.

So .... If you or your students find something that is significantly
overstated and/or underexplained, please let me know. I'll fix it
if I can.


I make a big fuss about "hard" versus "easy" in the context of
special relativity because I see a lot of bad horrible no-good
pedagogy in this area. Specifically, I'm talking about the so-
called "paradoxes". As far as I can tell, there are no paradoxes
in the correctly-stated laws of special relativity. The only way
to set up a paradox is to mis-state the laws of physics. I want
to tear my hair out every time I see that. There are some teachers
out there that positively revel in making relativity seem weird,
paradoxical, unnecessarily complicated, and disconnected from
everyday life -- which is almost 100% wrong.

I'm not saying SR is easy like falling off a log, but I insist it
is a whole lot easier than many texts and many teachers make it
out to be.

Also: Generally speaking, it's easier to explain SR to high school
students than to high school teachers. That's because the latter
have learned the wrong way of doing things, based on the pre-1908
way of looking at things, and unlearning all that is incomparably
more difficult than just doing things the easy way from the get-go.

If you look at my screed
you may be amused to note that:
-- It does not even mention FiztGerald-Lorentz contraction.
-- It does not even mention time dilation.
-- It does not even mention velocity-dependent mass.
-- It does not even mention rest mass.

It's not like a I found an easy way to explain those things; the
point is that you don't need those things. Not ever. Not at all.
A naïve high-school student knows that when you rotate a ruler in
the xy plane, its length does not change. So if you tell him that
when you rotate the same ruler in the xt plane, its length does
not change, there is no explaining necessary. He looks at you and
says "Well, duh." It is only the poor guy who learned the pre-1908
way of doing things that needs an explanation.

Please, folks, let's stop teaching stuff that is more than 100
years out of date.

If you want to read about the hard way of doing things, I have a
separate document for that ... but please keep in mind that these
are ideas you should not share with the students. It would just
pollute their brains.

This makes it hard to publish a book that contains what students
need. That's because the students don't choose the books. The
explanation that makes sense to the student doesn't make sense to
the teacher.

That's one nice thing about publishing on the web. Students have
ways of finding what they need. Sometimes the teacher finds out
about it from the students.
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