Re: [Fhsst-physics] False Information in an example?

[Jaynie Padayachee]

Dear Lucas,

Thank you for pointing out the error in the worked example and it
is great that you are finding the book useful.

However, there are a few things that you need to be aware of:

1. The books are not complete and are  therefore not ready to
use.

2. It appears that you are accessing the old FHSST site and
therefore a very old version of the book.

3. The new site is at www.fhsst.org where the latest versions of
the books can be found.

Jaynie
On behalf of FHSST Admin Team


Date sent:              Sat, 21 Oct 2006 12:01:12 +0200
From:                   Lucas Vogelsang <address@hidden>
To:                     Markus Oldenburg <address@hidden>
Subject:                Re: [Fhsst-physics] False Information in an
example?
Copies to:              FHSST Physics discussion list <fhsst-
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Send reply to:          FHSST Physics discussion list <fhsst-
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        <mailto:address@hidden>

> Hi
>
> Ok, thanks!
>
> I am actually learning for an ordinary exam. About electronics and
> magnetism. I was just a little too lazy during the lessons to write down
> what the teacher said. I am going to the Gymnasium Hohe Promenade in Zurich.
>
> I am half american so reading & writing in english isn't a big problem.
>
> Why I wrote "even though I am from Switzerland". At our school we have
> to pay for the books. I bought a physics book, but I don't really find
> it helpfull. The book you wrote, teachs physics exactly in the way my
> teacher did and I find it quite helpful.
>
> I think one should publish this book not only in South Africa. We have
> to pay about 400 CHF(roughly 300$) each year for books. For most of the
> pupils, this isn't a problem, but I still think, that it is a waste and
> could be cheaper if we were using open contents.
>
> Je préfère écrire en anglais, c'est plus facile. :)
>
> Hope you have fun at the CERN. You're have already several more years of
> education on your  back. :) I hope I'll be able to work at such an
> institute as well.
>
> Greetings from Zurich
> lucas
>
> Markus Oldenburg wrote:
> > Hello Lucas,
> >
> > thanks for coming back to us with your findings. It is great to hear
> > that you can make good use of the book 'even in Switzerland'. ;-)
> >
> > I checked the worked example 75 and you are right. There is an error
> > with the sin and the cos. Unfortunately there is another subsequent
> > error, which renders the whole solution wrong (numerically). We will
> > change the book accordingly.
> >
> > For now here is how it is supposed to be:
> > [Remark: All the text stays the same (except for the last sentence),
> > just the equations change.]
> >
> > First change: Step 3
> > F_E = T * cos (60 degrees)      F_g = T * sin (60 degrees)
> > This is the error you spotted.
> > Explanation: T is the 60 degree projection on F_E, therefore it has to
> > be the cos. On F_g instead T is the 30 degree (90 degrees - 60
> > degrees) projection, which leaves us with cos (30 degrees) = sin (60
> > degrees).
> >
> > Second problem: Step 3, second equation:
> > This equation is mathematically correct, but the result is wrong. It
> > should be T=115.5 N (not 1155 N).
> >
> > Subsequently, step 3, third equation (the cos (60 degrees) was and is
> > correct here, the typo happened earlier as mentioned above):
> > Putting the correct result for T from the last equation here gives us:
> > F_E = 115.5 N * cos (60 degrees) = 57.75 N.
> >
> > Finally, step 4:
> > |Q_x| is numerically wrong, due to the error introduced earlier. The
> > correct value is:
> > |Q_x| = sqrt( F_E * r^2 / k) = sqrt ( (57.75 N) * (0.5 m)^2) / ( 8.99
> > * 10^9 N * m^2 / C^2) )
> >          = 5.67 * 10^(-5) C
> > "This the charge on X is -5.67 * 10^(-5) C."
> >
> > Thanks again for finding this inconsistency. Please let me know if
> > everything is clear to you now.
> > What physics test are you learning for? Where in Switzerland are you?
> > You can also contact me in German (not in French, yet ;-) ) if this is
> > easier for you.
> >
> > Greetings from Geneva,
> > Markus
> >
> > address@hidden
> >
> > Lucas Vogelsang wrote:
> >> Hi,
> >>
> >> I have found your book via Wikibooks. I am currently learning for a
> >> physicstest and think that this book is quite helpfull for me, even
> >> though I am from Switzerland.
> >>
> >> However one thing confused me:
> >>
> >> The worked example 75 seems to be wrong.
> >> It first says:
> >>
> >> F_{E} = T*sin(60) and F_{g} = T*sin(60)
> >>
> >> next time they refer to F_{E} as F_{E} = T* cos(60) = ...
> >>
> >> So what is correct?
> >>
> >> I have found this in a pdf. It is on the pdf page 230 or the page 222
> >> according to the documents page numbers.
> >>
> >> regards,
> >> Lucas
> >>
> >>
> >> _______________________________________________
> >> Fhsst-physics mailing list
> >> address@hidden
> >> http://lists.nongnu.org/mailman/listinfo/fhsst-physics
> >>
>
>
>
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