# Rest mass time dependent?

1. Jun 30, 2015

### ChrisVer

I was wondering... can the rest mass of an object be time dependent? Like in a scenario where the body is losing mass?

(Sorry I meant for a title "rest mass time dependent?")

2. Jun 30, 2015

### Orodruin

Staff Emeritus
Only if the object is losing mass by somehow radiating it away.

3. Jun 30, 2015

### HallsofIvy

Staff Emeritus
Or if you chop pieces off with a knife!

4. Jun 30, 2015

### Staff: Mentor

Last edited: Jun 30, 2015
5. Jun 30, 2015

### Matterwave

I think the usual application of this scenario is the rocket problem. :)

6. Jun 30, 2015

### stedwards

Well, this is the relativity subforum, where rest mass is synonymous with energy. What process doesn't change the energy?

The energy of a tree changes if I start walking to the left.

7. Jun 30, 2015

### Staff: Mentor

No, it isn't. Energy is one component of the 4-momentum vector, and is frame-dependent. Rest mass is the invariant length of the 4-momentum vector, and is not frame-dependent.

8. Jul 1, 2015

### Ibix

Using older terminology than Peter, you're talking about relativistic mass (a concept that has largely fallen out of favour, by the way), not rest mass.

9. Jul 1, 2015