# Inverse Square Law problem

1. Feb 22, 2007

### antiduh

Howdy folks, I'm trying to solve a physics problem that I posed to myself one day after a class. My inspiration is a linear particle accelerator.

So, what I want to do is model a particle in one dimension, being attracted by an inverse-square-law force.

Code (Text):

|<---------d-------->|
|<--x-->|            |
|       |            |
|-------O------------M
|       ^            ^
|            |
Projectile           |
|
Attracting point source
Object 'M' is the attracting point source, such as a magnet. Object is fixed.
Object 'O' is the object being attracted, obviously free to move.

Distance 'd' is the position of M.
Distance 'x' is the posiition of the projectile.

So, let us assume that the force on 'O' is:
Code (Text):

S
F = -------
(d-x)^2
Value S is a unitless abstraction of field strength.

The problem is thus: Find a closed-form solution for the position of the object at time 't'.

Any help would be appreciated. I'd prefer someone to just point me in the right direction, and not solve the whole thing for me, but I'll take whatever help I can get.

Last edited: Feb 22, 2007
2. Feb 22, 2007

### stunner5000pt

i have lost touch with how to do this but i think you do this

find the potential first $$V = \int F \cdot dl$$

then find the Lagrangian L = T - V

Use Euler Lagrange equation to solve for this trajectory

but i could be wrong...

3. Feb 22, 2007

### stunner5000pt

an easier way is

$$F = m \frac{d^2 x}{dt^2} = \frac{S}{(d-x)^2}$$

you now have a linear second order DE which you need to solve