The expression relating force and acceleration for a particle with constant non-zero rest mass m moving in the x direction at velocity v is: F = ( γ γ 3 m a x , γ γ m a y , γ γ m a z ) , where γ γ = 1 1 − − v 2 / c 2 . is called the Lorentz factor. The Lorentz factor increases steeply as the relative velocity approaches the speed of light. Consequently, the greater and greater force must be applied to produce the same acceleration at extreme velocity. The relative velocity cannot reach c . If v is very small compared to c , then γ γ is very close to 1 and F = m a is a close approximation. Even for use in relativity, one can restore the form of F μ μ = m A μ μ through the use of four-vectors. This relation is correct in relativity when F μ μ is the four-force, m is the invariant mass, and A μ μ is the four-acceleration.