How this calculator works
Kinetic energy is total relativistic energy minus rest energy. It approaches mv²/2 at low speed and grows without bound as a massive particle approaches c. The computation evaluates gamma-1 using an equivalent stable expression to preserve low-speed energies. Mass means invariant rest mass. The mass inverse needs non-zero motion and energy; zero kinetic energy at zero speed cannot determine mass. Photons require their own massless energy-momentum relation.
Formula
K = (gamma-1)mc²; Etotal = gamma mc²; gamma = 1/sqrt(1-v²/c²). For inversion gamma = 1+K/(mc²).
Worked example
For a 1 kg mass moving at 0.6c, gamma is 1.25 and kinetic energy is about 2.246887947e16 J. At low speed the value approaches the classical result.
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Frequently asked questions
Is rest energy included in kinetic energy?
No. Rest energy mc² is shown separately, and total energy is their sum.
Why not use relativistic mass?
Invariant rest mass keeps the energy and momentum equations consistent without introducing a speed-dependent mass convention.
Can zero speed determine mass from energy?
No. Every rest mass has zero kinetic energy at zero speed.