# Mv and mv2

1. Sep 9, 2011

### keepit

are momentum and energy relative?

2. Sep 9, 2011

### Ken G

Yes, they both depend on the reference frame. The quantity that can be built from them that is not relative is called the "rest mass", and that is an invariant-- something that is not relative.

3. Sep 9, 2011

### keepit

thanks Ken G.
Re: conservation of energy. Does that conservation law apply only to rest mass energy.? It seems like the accounting for conservation of energy involving moving mass would be very complicated.

4. Sep 10, 2011

### khemist

If I understand correctly, the entire system will obey conservation of energy, regardless of velocity.

5. Sep 10, 2011

### Ken G

Just to be clear, the conservation of energy is a rather different issue from how relative energy is. Conservation laws mean you have something that stays the same with time when seen from a single inertial frame, but invariance means something that stays the same when the same event is reckoned from different reference frames. The conservation law applies to all sources of energy, including rest mass if it changes, for any closed system. It comes from the fact that a closed system should have no externally-imposed dependence on time, and the basic laws of dynamics then tell you how to construct an energy concept that will be conserved.