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

[Chewy0087]

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 = $$\frac{M}{AD}$$

Which i put in the Vmax equation giving me; Vmax = $$\frac{BM}{AD}$$

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

Emax = $$\frac{pA^2B^2M^2}{2mA^2D^2}$$ which i canceled to give me Emax = $$\frac{pB^2m}{2D}$$ is this correct? :F

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

Emax = $$\frac{2 * 10^-11 * 4 * 10^14}{2*10^3}$$ 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!

 

[Mark44]

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 = $$\frac{M}{Ad}$$

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).

Which i put in the Vmax equation giving me; Vmax = $$\frac{BM}{Ad}$$

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

Emax = $$\frac{pA^2B^2M^2}{2mA^2D^2}$$ which i canceled to give me Emax = $$\frac{pB^2m}{2D}$$ is this correct? :F

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

Emax = $$\frac{2 * 10^-11 * 4 * 10^14}{2*10^3}$$ 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!

 

[Chewy0087]

yeah it was capital D i meant, I've cleared it up a bit now