# Homework Help: Question on Elliptic Orbits. Difficult.

1. Mar 19, 2013

### Wesc

A satellite undergoes an elliptic orbit about the Earth of mass M, with maximum
distance 6R and minimum distance 3R from the Earth's centre.

(a) Show that twice the minimum velocity v(min) = the maximum velocity v(max) = (2/3)*sqrt(GM/R)

(b) Show eccentricity = 1/3

We are told that we can assume the orbit is described by 1/r = (1 + e*cos(Theta))/L where where r is the distance from the Earth’s centre, e is the eccentricity with 0 ≤ e < 1
and l = h^2/GM for constant h, the angular momentum per unit mass

How I started off was using the conservation of angular momentum and from that got v(max) = 2*v(min) . Tried conservation of energy but got nowhere there. Also tried other equations but I'm not getting anywhere! If anyone could show me a solution I would be so grateful :) Thank you.

2. Mar 19, 2013

### voko

Can you show exactly how conservation of energy got you nowhere?

For (b), what are the min/max values of the RHS of the equation of motion you were given?

3. Mar 19, 2013

### Wesc

Hey, thanks for the feedback.

Well for conservation of energy I did this:

E = (1/2)*m*v(max)^2 - G*M*m/r(min) = (1/2)*m*v(min)^2 - G*M*m/r(max)
I then cancelled the 'm's .. and I'm not sure if you can really extract any other information from there??

And if you're talking about this equation of motion 1/r = (1 + e*cos(Theta))/L then the max is at cos(Theta) = 1 and min at cos(theta) = -1 but I'm just not sure what the next step is? I'm just a bit lost. :/

4. Mar 19, 2013

### voko

You have obtained the relationship between Vmax and Vmin, and you were given the relationship between Rmax and Rmin. Use them.

That is correct, plug that into the equation and use Rmax and Rmin.