# Questions on classical mechanics

by Gavroy
Tags: classical, mechanics
 P: 235 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?
Homework
HW Helper
Thanks
P: 12,475
 Quote by 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.
To be more precise: the sun and the planet each orbit their common center of mass.
 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.
But it is still conserved.
 so how do i found out how to choose this point, if the system is more complex?
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?

 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?
Kinetic energy is conserved in an elastic collision: is the collision an example of a conservative force field?
P: 235
 Quote by Simon Bridge Is it not always the center of mass for gravitating systems?

Homework
HW Helper
Thanks
P: 12,475
Questions on classical mechanics