# Choice of reference frame in conservation of momentum/energy problems

Problems related to conservation of momentum/energy can be solved by taking reference frame which is stationary to center of mass or which is stationary to ground/observer. So when to use what frame or which frame is beneficial in which case/type of problems? I m confused ! ## Answers and Replies

AlephZero
Science Advisor
Homework Helper
If you are talking about Newtonian mechanics, you can use any inertial reference frame you like.

If there are no external forces acting on the system, the CM moves with constant velocity, so you can use an inertial frame where the CM is stationary. For example in simple collision problems this is very nice, because the velocities of two approaching objects must be +p/m1 and -p/m2 where p is the momentum of one of the objects, and for a collision with a coefficient of restitution e, the velocities after impact are -ep/m1 and +ep/m2.

That is much simpler than working in a different reference frame.