The equations of Newtonian physics are not entirely accurate. They're approximations. In everyday scenarios, the differences between the answers they give and the "actual" answers are negligible, so we take them as correct. But for an extreme scenario - an extremely high-gravity environment, or a sub-atomic scale, or for an object moving at an appreciable fraction of the speed of light - the difference between the Newtonian approximation and the truth is so massive the equations are entirely useless. So in your hypothetical situation, [i]F = ma[/i] goes out the window. In fact, seeing as you're talking about above-light speeds, I doubt the equations of special relativity or even M-theory would apply. So we really have no way of knowing what would happen. I also don't think you can claim with any certainty that an above-[i]c[/i] particle would have an infinite mass.