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EinsteinMass
See how mass increases as an object approaches the speed of light.
In special relativity, an object's relativistic mass increases with velocity: m = m₀ × γ, where γ = 1/√(1 − v²/c²) is the Lorentz factor. At 90% of light speed, γ ≈ 2.29, so the object's effective mass more than doubles. At 99.9% c, γ ≈ 22.4. This is why no massive object can reach the speed of light — it would require infinite energy.
Modern physics prefers 'invariant mass' (rest mass m₀), which doesn't change with speed. 'Relativistic mass' is a pedagogical concept showing how inertia grows with velocity. The energy relationship E = γm₀c² is the precise formulation.
Relativistic mass
2.294 kg
What you entered
v/c (velocity as fraction of speed of light)
0.9= 0.9cv² / c²
0.9²= 0.81Lorentz factor: γ = 1 / √(1 − v²/c²)
1 / √(1 − 0.81)= 2.2942Relativistic mass: m = m₀ × γ
1 × 2.2942= 2.2942 kgResult
Relativistic mass: 2.294 kg
At 90.0% the speed of light, the Lorentz factor is 2.294, so a 1 kg rest mass appears as 2.294 kg to a stationary observer.