Re: Question about Kepler's second law

On Apr 16, 5:49 pm, mme...@xxxxxxxxxxxxxxxxxx wrote:
In article <1176754884.114148.59...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>, "Peter" <Poakfi...@xxxxxxx> writes:
On Apr 16, 3:41 pm, "PD" <TheDraperFam...@xxxxxxxxx> wrote:
On Apr 16, 8:06 am, "Peter" <Poakfi...@xxxxxxx> wrote:

On Apr 14, 8:48 pm, "PD" <TheDraperFam...@xxxxxxxxx> wrote:

On Apr 14, 6:42 pm, "Peter" <Poakfi...@xxxxxxx> wrote:

On Apr 14, 6:54 pm, Phineas T Puddleduck <phineaspuddled...@xxxxxxxxx>
wrote:

In article <1176590209.784235.154...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,

"Peter" <Poakfi...@xxxxxxx> wrote:
Angular velocity is a vector too, hence the cross product

(Point of contention - -actually a pseudovector, but it still uses the
cross product)

- Show quoted text -

The cross product is also a pseudovector.

Peter

Are you trolling, or deliberately obtuse?

You tell that to Douglas C. Giancoli (Physics for Scientists and
Engineers, 2nd ed., page 220).

Peter

Ah, well, at least you've moved up to an (old) college text. You'll
note that the terms "pseudoscalar" and "pseudovector" are not listed
in that freshman text. What does that tell you?

PD- Hide quoted text -

- Show quoted text -

Giancoli does talk about pseudovectors.

Peter

My error. I was looking at a different edition. Now, you will note
that Giancoli also tells you how a "pseudovector" is defined by its
properties under transformation. You'll note that it is not
distinguished as being a "false" or "not real" vector.

PD- Hide quoted text -

- Show quoted text -

I am not sure I understand what you mean. But I know that if something
is not a true vector, it is just not a vector. You know why angular
displacements are not vectors: they fail the commutative law for
addition.

Sigh. A clear waste of time.


Well, to be fair, this is the risk of introductory to sub-introductory
level material of the sort that Peter is trying to wrestle with.
Material like that has to skirt (at least) two hazards:
- it gives the reader the impression that he understands more than he
really does;
- it uses some jargon flippantly and without fully explaining the
meaning of those terms, leaving the reader to (usually wrongly) fill
in the gaps.
Writers of material like that are playing with fire for a reason. They
have to engage the interest of the novice, and they can't subject the
casual reader to the full-blown exposition that would produce a better
understanding.

The problem is that, while most novices know they're getting only part
of the picture, folks like Peter want to be able to work with the
introductory material as sufficient for full understanding, and it's
tremendously frustrating to learn that this just isn't possible.

PD

.

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