# Homework Help: Deriving Vr and V(sub-theta) from Kepler's second law and

1. Sep 24, 2010

### LordCalculus

Deriving Vr and V(sub-theta) from Kepler's second law and....

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

Beginning with r=[a(1-e2)]/(1+e*cos $$\theta$$) and Kepler's second law, derive general expressions for vr and v$$\theta$$ for a mass m1 in an elliptical orbit about a second mass m2. The final answers should be functions of P, e, a, and $$\theta$$ only.

P=orbital period
e=eccentricity
a=semimajor axis

That's supposed to be v(subtheta) and cosine of theta!

2. Relevant equations

A'(t)=L/(2*$$\mu$$)

2. Sep 24, 2010

### diazona

Re: Deriving Vr and V(sub-theta) from Kepler's second law and....

$v_\theta$, understood. What have you done so far and where exactly did you get stuck?

3. Sep 26, 2010

### LordCalculus

Re: Deriving Vr and V(sub-theta) from Kepler's second law and....

Not far at all. I don't even know where to begin. If you do, then please start me off. Thanks!

4. Sep 26, 2010

### diazona

Re: Deriving Vr and V(sub-theta) from Kepler's second law and....

Well, here would be my first thought: you have some information about position, and you are asked to find velocity. What does that tell you?