1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Text book solution

  1. Apr 18, 2012 #1
    1. The problem statement, all variables and given/known data
    please look at equations II.2.8 , II.2.9, II.2.10 in the attached scan,

    2. Relevant equations

    3. The attempt at a solution
    im unable to get from II.2.8 to II.2.9, i wonder where they got the extra r in the denominator of the stuff in the brackets,
    here is what i get for II.2.10:
    [tex] \frac{1}{18} \left( \frac{ (n+2) n C} { r^{n+3}} - \frac{3 M e^2}{r^4} \right) [/tex]
    what im i doing wrong?

    Attached Files:

  2. jcsd
  3. Apr 18, 2012 #2

    rude man

    User Avatar
    Homework Helper
    Gold Member

    Looks like sloppy work to me:

    1. in going from II.2.8 to II.2.9 somehow E changed to e.

    2. There should be a minus sign in front of II.2.9, otherwise it's correct.

    remember: d/dr(rn) = nrn-1 etc.

    No way I can determine the correctness of II.2.10 since a new parameter (r0) is introduced.
  4. Apr 19, 2012 #3
    how did u do that? in II.2.8 there is (for stuff after partial differentiation***{EDIT})
    [tex] \frac{1}{6r^2} \left( ME^2 \frac{1}{r} - \frac{nC}{r^n} \right) [/tex]
    how does this end up being
    [tex] \frac{1}{6} \left( ME^2 \frac{1}{r^4} - \frac{nC}{r^{n+3}} \right) [/tex]
    when the stuff just outside the bracket gets taken in? r is only to the inverse second power, why does r in II.2.9 end up to the inverse fourth power? 2+1 =4 ? remember the partial diff hasn't yet been done.

    i mean partial differentiation sign not after performing the operation.
    Last edited: Apr 19, 2012
  5. Apr 19, 2012 #4
    though i think the [itex] r_o [/itex] is the r corresponding to minimum P.E., looking at II.2.7 and the paragraph preceeding it.
  6. Apr 19, 2012 #5

    rude man

    User Avatar
    Homework Helper
    Gold Member

    Oops - I hadn't noticed - in II.2.9 there should not be a partial differential anymore. The derivative is taken already in going from II.2.8 to II.2.9.

    Like I said - sloppy work! And there still needs to be a minus sign in front of the corrected II.2.9.
  7. Apr 19, 2012 #6
    Ok, i see that now, i had not noticed that the differentiation had already been done, so that's why you kept on insisting on the negative sign, ( i admit i was clueless why until now).
    thanks. Let me work on it and see whether i can better this guy's work.
  8. Apr 19, 2012 #7
    one quick last one, has anyone seen this question before? is n=8.1 really? is that the correct answer that i need to find when i correct this guy's unholy mess?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook