# Length contractions affect on mass

1. Dec 28, 2013

### NotAMethDealer

If an object is going at near C(light velocity) and undergoes length contraction. Does the mass fluctuate?I apologize if it is an obvious question but I'm a little rusty on my physics.

2. Dec 28, 2013

### bcrowell

Staff Emeritus
This is more a matter of convention than a matter of fact. The older convention was to say that mass would go up by a factor of gamma: http://en.wikipedia.org/wiki/Relativistic_mass#Relativistic_mass In that convention, momentum was still p=mv. Since about 1950, relativists have no longer used that convention. They take mass to be invariant and write $p=m\gamma v$. Either way, it's not an effect caused by length contraction.