In Newtonian mechanics the mass of a particle is constant and can be expressed as the ratio of the force to the acceleration:
The force can be written as the rate of change of momentum:
so
In Newtonian mechanics
and
is constant so
In special relativity, mass plays more of a resistance to acceleration role, and has a different value in the direction of acceleration than at right angles to it.
In special relativity the momentum of a particle of rest massis
where
If the acceleration is perpendicular to
then 
and
If the acceleration is in the direction of
