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Algebra manipulation

  1. Sep 8, 2009 #1
    1. The problem statement, all variables and given/known data
    sorry for another one of these threads...hopefully after this one i wont have to bug anyone else xS

    http://www.physics.ox.ac.uk/admissions/undergraduate/admissions_test/tests08.pdf [Broken]

    page 6 - Question 23 - ONLY HELP WITH THE SECOND HALF I NEED, the 'SHOW THAT...' BIT IS FINE

    2. Relevant equations

    mass = A * thickness * density

    3. The attempt at a solution

    this is simply more of a checking thing, I know i should have more confidence and stuff and i know i find these threads annoying but i'd really appreciate just some confirmation that i have indeed got it right!

    basically I did M = AdD (with m bieng mass and D bieng density)
    rearranged to get d = [tex]\frac{M}{AD}[/tex]

    Which i put in the Vmax equation giving me; Vmax = [tex]\frac{BM}{AD}[/tex]

    I then proceeded to put that V max and d figures into the main equation giving me;

    Emax = [tex]\frac{pA^2B^2M^2}{2mA^2D^2}[/tex] which i cancelled to give me Emax = [tex]\frac{pB^2m}{2D}[/tex] is this correct? :F

    the reason why I worry is that I put the numbers they give into the equation to give me;

    Emax = [tex]\frac{2 * 10^-11 * 4 * 10^14}{2*10^3}[/tex] which cancels to give only 4J....

    if that is correct what would that imply for the final question about it's practicality? i guess it would not be practical because it cannot store enough energy?

    thanks for the help again guys!
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Sep 8, 2009 #2

    Mark44

    Staff: Mentor

    Is this problem 23? That's the only one on p. 6 that seems remotely close to what you've written above. In your first equation you say that D is density, but what is d? In your rearrangement, you have not solved for d (whatever it represents), since there is a d factor still on the right side. Did you mean to write D instead of d? If you were solving for D, the new equation would be D = M/(Ad). Or if you were solving for d, the new equation would be d = M/(AD).
     
    Last edited by a moderator: May 4, 2017
  4. Sep 8, 2009 #3
    yeah it was capital D i meant, i've cleared it up a bit now
     
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