Algebra Manipulation Homework: Finding Emax and Vmax in Mass-Density Equation

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Chewy0087
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Homework Statement


sorry for another one of these threads...hopefully after this one i won't have to bug anyone else xS

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

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

Homework Equations



mass = A * thickness * density

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 canceled 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:
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Chewy0087 said:

Homework Statement


sorry for another one of these threads...hopefully after this one i won't have to bug anyone else xS

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

page 6

Homework Equations



mass = A * thickness * density

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]
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).
Chewy0087 said:
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 canceled 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:
yeah it was capital D i meant, I've cleared it up a bit now