# Questions on classical mechanics

1. Apr 8, 2013

### Gavroy

Hi
Question 1
i was wondering if there is any method to find the point from which the angular momentum is a conserved quantity. let me e.g. choose the case of a planet moving in an orbit around the sun. In this case, the angular momentum measured from the center of mass as the point of reference is conserved. But if i choose a different point, that is not exactly on the line connecting the planet and the sun, then the angular momentum is varying with time. so how do i found out how to choose this point, if the system is more complex?

Question 2
if F is a conservative force field, then the energy is conserved. is the converse, the energy is conserved, therefore we have a conservative force field also true?

2. Apr 9, 2013

### Simon Bridge

To be more precise: the sun and the planet each orbit their common center of mass.
But it is still conserved.
You mean you want to find a reference frame in which a particular bodies angular momentum is a constant?

Is it not always the center of mass for gravitating systems?

Kinetic energy is conserved in an elastic collision: is the collision an example of a conservative force field?

3. Apr 9, 2013