# Rest mass

#### kur82

Hi all. we know that the rest is the mass of an object within a coordinate system which has zero velocity, but how does it exist for an electron inside a crystal, which we call rest mass of electron, how electron exist with zero velocity.or we mean by zero velocity of electron with respect to an observer?

#### ZapperZ

Staff Emeritus
2018 Award
Hi all. we know that the rest is the mass of an object within a coordinate system which has zero velocity, but how does it exist for an electron inside a crystal, which we call rest mass of electron, how electron exist with zero velocity.or we mean by zero velocity of electron with respect to an observer?
Try giving an electron a KE of a few eV. Now, try and calculate how much relativistic effects comes into play at such energies. Do you think it makes that much of a difference for a mass that we measure at such energies with the "rest mass"?

Furthermore, inside a solid, the "effective mass" due to the many-body interactions has a greater effect on the "mass" of an electron (or quasi-electron) than the issue of relativistic mass in many cases.

Zz.

#### Matterwave

Gold Member
When regarding mass, it's simpler and better to just regard the whole energy/mass of a bounded system together instead of trying to index them element by element. For example, the mass of a proton (~900MeV/c^2) is not equal to the mass of the three quarks (~2-5MeV/c^2) which compose it, but mostly due to the strong interaction between the quarks.

Practically speaking this is what we do anyways when we put an object onto a scale for measurement.

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