Derive Angular Momentum in Elliptical Orbit

[psychicist]

Homework Statement

A planet with mass m is orbiting about the Sun (mass m(s)) with elliptical orbit such that the maximum and minimum distance is r1 and r2 respectively. What is its angular momentum, L, of the planet relative to the centre of the Sun?

Homework Equations

\vec{L}=\vec{r}\times\vec{p}=m(\vec{r}\times \vec{v})=mrv sin\theta
Semi-major axis, a=(r_1+r_2)/2
Semi-minor axis, c=(r_1-r_2)/2
Distance of the planet from the Sun, R=\frac{a(1-e^2)}{1-e cos\theta}
e=c/a

The Attempt at a Solution

Known that the angular momentum is conserved along the motion, but I am confused about how to relate the variables, i.e. r and v.

I have tried to derive R and tried to put into the equation of angular momentum but the formula seems make it more complicated as it introduce one more variable theta inside.

Can someone give me some idea? Thank you very much.

 

[nasu]

Try to use conservation of energy (kinetic plus potential) between the two points.
Write the speed in terms of the angular momentum:
L=m*v1*r1=m*v2*r2
Then solve for L.

 

[psychicist]

Try to use conservation of energy (kinetic plus potential) between the two points.
Write the speed in terms of the angular momentum:
L=m*v1*r1=m*v2*r2
Then solve for L.

Yes! I got it! thank you very much!

 

[nasu]

You are welcome.