# Mass in SR

1. Aug 18, 2009

### elec-tric

is the mass invariant in special relativity or what ?
i'm so confused , is the mass of a body change with the change of the body's speed ?

2. Aug 18, 2009

### Fredrik

Staff Emeritus
Two different conventions are being used. The one favored by most people today is this one:

Mass is invariant. It's denoted by $m$. The quantity

$$\frac{mc^2}{\sqrt{1-\frac{v^2}{c^2}}}$$

is called "energy". If we have to use a name for the quantity

$$\frac{m}{\sqrt{1-\frac{v^2}{c^2}}}$$

we call it "relativistic mass", but we prefer not to use it at all.

The other convention is to use the notation $m_0$ and the term "rest mass" for what I would call "mass", and the notation $m$ for what I would call "relativistic mass". This convention makes "mass" a function of speed:

$$m=\frac{m_0}{\sqrt{1-\frac{v^2}{c^2}}}$$

3. Aug 18, 2009

### ZapperZ

Staff Emeritus
Please read an entry in our FAQ in the General Physics forum.

Zz.