mass dosent increase when in motion?, because as the acceleration increases so does the force, since they are proportional so the resultant mass is the same e.g.
rest mass = 0.5kg and a force of 5N downwards when accelerated downwards at 12 ms instead of 9.81ms the force will also increase by a factor of the acceleartion increase, so new m =6.11/12 = 0.5kg