It doesn't matter if you use relativistic mass. My point is that
at low speeds you should get F=ma. Because at low speeds, the effects of relativity (as well as any other new theory that may replace it) are negligable.
So what's the relativistic equation for force?
If you plug in the relativstic mass:
M=(m/ (1-(v ^2 /c ^2 ))^(1/2)
in the formula F=Ma,
You'll get:
F=[(m/ (1-(v ^2 /c ^2 )) ^(1/2)]a
Which at low velocities (v<<c) reduces to F=ma.
Sounds right? Well, it isn't. Because F=Ma is valid only when M is constant.
Here is the equation for relativistic force, as given in wikipedia:
The first term is indeed equal to Ma (that funny looking symbol before the "m" is the greek letter gamma, which is shorthand for 1/(1-v^2/c^2)^(1/2)). The second term stems from the fact that M is not constant.
Not surprisingly, this equation also reduced to F=ma at low velocities.