# Relative energy of simple 2 particle system (confused )

1. Oct 3, 2011

### Jonnyb42

Relative energy of simple 2 particle system (confused!!)

Ok so I have a simple question which I feel I should know the answer to:

Setup:
Two particles of different mass.. say M and m (where M > m) are moving past each other by some constant velocity. If we view the energy of the system from m.. or fix our reference on m, we observe some energy of the total system.

Why is the energy of the system dependent on which particle we use as reference??
Shouldn't the energy of the system be the same regardless of reference frame?

(What I mean is, the energy is the total potential, plus the kinetic of the one that is moving. This energy is greater for the smaller mass, because it sees M moving with K = 1/2 M v^2, while the mass M sees m moving with K = 1/2 m v^2)

thanks

2. Oct 3, 2011

### Staff: Mentor

Re: Relative energy of simple 2 particle system (confused!!)

Physics homework questions should be posted to the physics homework forum.

3. Oct 3, 2011

### Jonnyb42

Re: Relative energy of simple 2 particle system (confused!!)

..this isn't a physics homework question.. just a wonder of mine

4. Oct 3, 2011

### aeroguy77

Re: Relative energy of simple 2 particle system (confused!!)

If your reference is attached to m, than that means you see the particle M traveling with kinetic energy 1/2Mv^2 while particle m has zero velocity, relative to M. If you were attached to M, than you would see the opposite. You would see kinetic energy 1/2mv^2. It's all relative brotha. I think you're being confused with the first law of thermodynamics, which states that energy cannot be created or destroyed. In this case it isn't. You're simply viewing energy from different perspectives.

Although, if you were an observer at rest watching the system of two particles travelling in opposite directions with M>m at constant velocity, what would you see?