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: 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


    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook