Re: moment of inertia of a cube

On Apr 12, 4:11 pm, "Bill Hobba" <rubb...@xxxxxxxx> wrote:
"PD" <TheDraperFam...@xxxxxxxxx> wrote in message

news:1176417859.932837.302910@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx



On Apr 12, 10:54 am, "Y.Porat" <y.y.po...@xxxxxxxxx> wrote:
and 22years imbecil that cant learn something new

there is no relativistic mass
there is only one kind of mass
and let pD answer let** human beings here discuss

There is little point in learning something new if
- it is not new
- it is not right
- it is not useful

To encourage someone to learn something new, you have to demonstrate
that
- it IS new
AND
- it IS right
AND
- it IS useful

This post is not exactly related to the topic, but other than starting a new
topic, I could not find a better one to mention it on. It relates both to
the advice PD gave me previously about study and Eric's comment that he
spends 40 hours on physics for his studies.

Even the one subject I am doing this semester is taking up enormous amounts
of time. I recently started looking at some past exam papers and found they
require quite a deal of long manipulations such as inverting a 3x3 symbolic
matrix (and that is only a small part of the problem) to solve. And guess
what - you must get one such question out without error to pass - I suppose
since it is masters level you got to expect things to be a bit tougher - but
this subject is also open to second year undergrads. I actually had a panic
attack the first paper I attempted and failed to solve. Only one solution -
a lot more practice at doing long manipulations. I can now do all questions
on all past papers - but if I can do it during an exam only time will tell.

There is a quasi-fast way of inverting a matrix.

Form the complementary matrix of A and I [identity matrix], then
reduce the left half into reduced row echelon form. Whatever is left
on the right hand side is A's inverse. Basically - whatever you do to
A to get to reduced row echelon form, you do to I. If you end up with
a row of zeros - you have a singular matrix and either you fucked up
or the problem has no solution.


I got my first assignment back the other day - not too bad - lost 1/2 mark
for a silly arithmetic error I should have double checked, another half for
not showing the full working of a step (despite the fact the lecturer made
this big song and dance about not putting in every step - I suppose I went a
little too far), and another half for not reducing an algebraic expression
as far as it would go. Not too bad I suppose - will try and do better next
time. The marker made a few comments that have got me stumped - I will need
to chat about it today with them at the tutorial. Learning a lot about
using LaTex though, which is a bit of fun.

I get burnt on silly *** as well. Today I found out I lost 10 points
[for reference - this set was 80 points, 4 problems] because I failed
to prove that the tensor I derived was a linear function, despite
having it in the form T = crap*(variable it was supposed to be linear
with respect to). That was half the problem in points but not anywhere
near half in actual effort. That earned a nice "What the hell?". I
think the professor just hates giving out A's for stuff. RAGE!

Little errors like that have a nasty way of propogating. "Oh, that
negative sign would have helped...". Personally, I make ~2 un-noticed
algebra errors/page before I go over the page. That was one of the
most helpful lessons this semester - I make errors and I know about
how many I make.

LaTeX is purty, but I don't have the patience to use it. I don't think
I will ever really use it until it is either expected of me or it
makes my life easier - two things which aren't true at all right now.
I can write math about as fast as I can think it, but I can think in
English a whole lot faster than I can write it - thoughts get backed
up and the result is me being pissed off.


Interestingly most students seem not to be math majors - mostly physics,
engineering, and computer science students.

Bottom line PD is your advice that the correct way to learn is going to uni
was spot on - I am way ahead of doing it from books.

Having someone to bug is invaluable. I have picked up a lot of GR
myself, but I _know_ there are lots of gaps that will be filled in
when I actually take a course or two in it. A lot of time would have
been saved if I actually had someone teaching the tensor stuff. Then
again, my classical mechanics course is cake right now because I
already have firm grip on tensors...


Bottom line Eric is I suspect you estimate eof 40 hours may be a bit
conservative.

True.

I have 15 course hours/week load [4 upper division courses - classical
mechanics, electrodynamics, solid state physics, modern physics], plus
3-ish more for tutorial [which I don't go to enough...]. What
_actually_ ends up happening is that the courses themselves eat up 21
hours/week [7 hours a day x 3 days, not including tutorial]. The 15
minutes between classes always gets eaten. I then spend easily that
much working on homework.

That doesn't even count the auxillary *** that I pile on - my time on
USENET isn't wasted because it either directs me to interesting papers
to read, or ends up getting combined with an interesting problem [Tom
Potter is a worthless person but calculating the GPS offset was a
worthwhile exercise] that chews up more time.


Thanks
Bill



PD


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