Thread: Plate dynamics (quadratic equation)

Hi, I've got a small problem with a quadratric equation I can't seem to be able to re-arrange to give me the correct answer. It's probably quite simple but I can't make it work!

So the following equation relates horizontal force (P) on a plate (as in tectonic plate) with it's elastic rigidity D, and the densities of mantle (rho_m) and water (rho_w) giving displacement w.

D(d^4w/dx^4)+P(d^2w/dx^2)+(rho_m-rho_w)gw = 0

by finding the 2nd and forth differentials of

w(x) = w_o * sin(2pi/lambda) *x

you can turn the top equation into:

D(2pi/lambda)^4 -P(2pi/lambda)^2 + (rho_m - rho_w)g = 0

Now, this is a quadratic in (2pi/lambda)^2. You're supposed to be able to rearrange this quadratic to give

P^2 >= 4D(rho_m-rho_w)g

but I just can't get there!

Any ideas? It's probably really simple but I've tried a hundred times and it just isn't clicking.

Thanks!

Originally Posted by chambochae

Hi, I've got a small problem with a quadratric equation I can't seem to be able to re-arrange to give me the correct answer. It's probably quite simple but I can't make it work!

So the following equation relates horizontal force (P) on a plate (as in tectonic plate) with it's elastic rigidity D, and the densities of mantle (rho_m) and water (rho_w) giving displacement w.

D(d^4w/dx^4)+P(d^2w/dx^2)+(rho_m-rho_w)gw = 0

by finding the 2nd and forth differentials of

w(x) = w_o * sin(2pi/lambda) *x

you can turn the top equation into:

D(2pi/lambda)^4 -P(2pi/lambda)^2 + (rho_m - rho_w)g = 0

Now, this is a quadratic in (2pi/lambda)^2. You're supposed to be able to rearrange this quadratic to give

P^2 >= 4D(rho_m-rho_w)g

but I just can't get there!

Any ideas? It's probably really simple but I've tried a hundred times and it just isn't clicking.

Thanks!

You're missing the point of what they want you to do here. You are to find a condition on the variables, not represent the equation:


This is, indeed, a quadratic in . So define . Thus


In order for this equation to have real solutions for z (which is required since I would presume that is a real number), we require the discriminant to be non-negative:


Your condition follows from this.

-Dan

Ah I see! Thanks ever so much!