# Is there mass without momentum?

1. Dec 4, 2011

### azabak

There's something called "rest mass" that is the mass of a body in "rest". But since rest is relative it is impossible to surely know something's mass. We also know that mass increses with velocity, so it seems possible that mass is nothing but a "shaded" speed in a certain referential.

2. Dec 4, 2011

### Zula110100100

In my bathroom I am at rest relative to myself and the scale. I thought there was a different way of looking at it that didn't require relative mass per se, but an understanding of mass-energy, objects don't get harder to accelerate due to increased mass if you say F = (mass-energy) * acceleration, then it is due to increased energy?

3. Dec 4, 2011

### Staff: Mentor

The name "rest mass" is a historical artifact and can be rather misleading. Physicists nowadays prefer to use the name "invariant mass." It can always be calculated from an object's momentum and energy using

$$E^2 = (m_0 c^2)^2 + (pc)^2$$

i.e.

$$m_0 c^2 = \sqrt{E^2 - (pc)^2}$$