# Energy in two different frames

1. Dec 12, 2011

### Lawrencel2

If you have a particle, and you know its rest mass is about 100 Mev/c^2 and in the lab frame you measure it to travel at (8/9)^1/2 c then what is its energy in the rest frame?? Would it be just 100MeV? or 300 MeV due to the relativistic constant? i don't really know how you would reason out the difference in frames? can anyone explain this idea?

2. Dec 12, 2011

### Staff: Mentor

In the particle's own rest frame, it's stationary, so its kinetic energy is zero. Therefore, in that frame, its energy is only the rest energy which is 100 MeV.

3. Dec 12, 2011

### kmarinas86

Keep in mind the so-called kinetic energy "of" a particle is really an energy associated with a collision between energetic particles. Acceleration with respect to a light source, causing a Doppler shift of the light being received, does NOT in anyway mean that the light energy is different relative to the motions of the observer, but rather, the energy measured once the light is received is of the collision between the photon and the observer. The same is true for particle (which in this case is not light). The additional "kinetic energy" supposedly either gained (or lost) by this particle by change of reference frame actually owes physical existence by the work done on the observer in the direction either toward (or away from) that particle, which is what will contribute additional energy to (or reduce the energy of) the collision.