[AndrewC]

F=ma is a great example of this. Great approximation at non-relativistic velocities, no good at all in the relativistic regime.

Well, maybe you can show the math.

[ThinkPlease]

Well, maybe you can show the math.

While Newton's second law generally holds, as you get close to the speed of light, there is a relativistic mass factor that becomes apparent as one gets closer to the speed of light, it is gamma, where gamma= (the rest mass energy+the kinetic energy)/the rest mass energy. So at high velocities f=ma becomes f=(gamma)ma. where gamma*m is the relativistic mass.