# Homework Help: Effective Potential

1. Apr 5, 2009

### roeb

1. The problem statement, all variables and given/known data

I am trying to do problem 5, I seem to be having a hard time with the algebra.
http://img24.imageshack.us/img24/4224/landau.th.png [Broken]

2. Relevant equations

3. The attempt at a solution

To find maximum value of the effective potential we just do: dU/dr = 0.
I get (which I have verified to be correct)
$$r_m^{n-2} = \frac{n*\alpha }{m \rho^2 v^2}$$

I rearrange that into
$$r_m = (\frac{n*\alpha }{m \rho^2 v^2})^{1/(n-2)}$$

Plugging that into the Ueff equation for r, I get (and simplifying a bit, let x = n/(n-2) ).

$$\frac{m^x v^{2x} \rho^{2x}}{2 n^{2x/n} \alpha^{2x/n}} - \frac{\alpha m^x v^{2x} \rho^{2x}}{n^x \alpha^x}$$

At this point I kind of get stuck, I can do the following, but it never seems to turn out 'right'.

$$\frac{m^x v^{2x} \rho^{2x}}{\alpha^x n^x} ( \frac{1}{2 \alpha^{2/n} n^{2/n}} - a)$$

Anyone know what I am doing wrong? it must be something simple because I am getting very close to the correct answer, it's just those terms on the right don't seem to become 'nice'.

Last edited by a moderator: May 4, 2017
2. Apr 5, 2009

### roeb

Whoops, I seem to have figured it out, apparently it's n^(-1) not n^2/n