# Lagrangian equations of motion

1. Oct 12, 2011

### Maybe_Memorie

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

Find equations of motion (eom) of a particle moving in a D-dimensional flat space with the following Lagrangian

L = (1/2)mv2i - k/ra,

r = root(x2i), m,k,a are constants

2. Relevant equations

3. The attempt at a solution

The equations of motion are given by d/dt(∂L/∂vi) - ∂L/∂xi = 0

So, when I work all this out I get
ma = ka/xia+1

I have a feeling this isn;t correct though.

Am I doing the partials wrong?

2. Oct 12, 2011

### vela

Staff Emeritus
Yes, you're taking the partial with respect to xi incorrectly. Try writing out the potential term in terms of the xi's and differentiating.

Also why is a multiplying m? How did the exponent of r get over there?

3. Oct 12, 2011

### Maybe_Memorie

U = k/ra = k/root(x2i)a
= k/xai
= kx-ai

Differentiating this with respect to xi gives -akx-a-1

As for the last part; sorry, when I wrote a on the LHS I was referring to the second derivative of xi w.r.t. time

4. Oct 12, 2011

### vela

Staff Emeritus
How are you going from k/root(x2i)a to k/xai?

Are you saying, for instance, that $\sqrt{x_1^2+x_2^2} = x_1+x_2$?

EDIT: Oh, I see why we're getting different answers. I think there's an implied summation: $$r=\sqrt{x_i^2} = \sqrt{x_i x_i} = \sqrt{\sum_i x_i^2}$$