# Homework Help: Central Forces related problem

1. Oct 14, 2015

### TimeLordo123

1. The problem statement, all variables and given/known data
The problem is related with central forces. In the problem I am given the equation of the orbit of a particle subjected to a central force (with an angular momentum "l"), r=a/(phi+1)^2 (where "r" is de distance to the center of forces and "a" a constant). I am asked for the potential energy of the particle, the force applied to it and then I am asked to discuss qualitatively the characteristics of the orbit of a particle subjected to this force, depending on its energy.

2. Relevant equations
E=(1/2)m(dr/dt)^2 + (l^2)/(2mr^2) + U(r)
F=-dU/dr

3. The attempt at a solution

Getting to the potential energy of the particle and force applied to it is rather easy, and I am sure the following expressions are correct: U(r)=E-(l^2/(2mr^2))(1+(4r/a)) and F=-(l^2/(mr^2))(2/a+1/r). I think the graph I am looking for is something like this: http://l27.imgup.net/adasdad3f46.jpg [Broken]. However now I don't know what levels of energy to use, and what to say about them. My guess would be that I have to simply use one positive and one negative level of energy.

Last edited by a moderator: May 7, 2017
2. Oct 19, 2015

### Greg Bernhardt

Thanks for the post! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?

3. Oct 20, 2015

### ehild

Your graph is the potential energy in terms of r, but you are asked about the graph of the orbit. How does it look like? What happens with the particle with increasing phi? Can it stay on a stationary orbit? What energy it should have then?

Last edited by a moderator: May 7, 2017