The resistive force gives you the equation of motion which is:
acceleration = v(dv/dx) = -v√(1 - v<sup>2</sup>)
dv/dx = - √(1 - v<sup>2</sup>)
dx/dv = -1/√(1 - v<sup>2</sup>) then integrate w.r.t. v to get
x = cos<sup>-1</sup>v + c ... when x=0, v=R so c =...