# I Why conservation of angular momentum is not applicable here

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1. Oct 7, 2016

### Mohammed Shoaib

Comets travel around the sun in elliptical orbits with large eccentricities. If a comet has speed 2.0×104 m/s when at a distance of 2.6×1011 m from the center of the sun, what is its speed when at a distance of 5.2×1010 m .

I applied conservation of angular momentum. But my answer goes wrong. Why?
my working

2. Oct 7, 2016

### rcgldr

Since the orbit is elliptical, the comet's velocity has a component in the direction of the sun (radial line) as well as a component perpendicular to a radial line to the sun. I'm not sure how you would determine angular momentum in this situation. You could use potential versus kinetic energy, but you'd have to know the mass of the sun (the comets mass, being much smaller than the sun's mass, could be ignored), and the problem statement doesn't include the mass of the sun.

Last edited: Oct 7, 2016
3. Oct 7, 2016

### Simon Bridge

The equation for angular momentum you used includes an assumption about the relationship between the velocity vector and the radial vector.
The relation you should use is $\vec L = \vec r \times \vec p$
[edit: beat me to it...]

4. Oct 7, 2016

5. Oct 7, 2016

### Mohammed Shoaib

Thanks for help.