Work out relativistic mass instantly with clear inputs, formula shown and shareable results.
Relativistic mass is gamma times rest mass, so a 1 kg object at 0.9c has an inertia of 2.294 kg. The extra inertia is really the kinetic energy: (gamma - 1)m0c^2, here 1.16e17 J. Modern practice keeps mass invariant and puts gamma in the energy and momentum instead, but the effect on accelerating the object is the same.
Relativistic mass
m = gamma m0
Kinetic energy
KE = (gamma - 1) m0 c^2
Because it invites confusion - it is direction-dependent for forces and adds nothing that energy and momentum do not already describe.
Because gamma diverges as v approaches c, so the energy needed grows without limit. Reaching c would take infinite energy.