At time t = 0, constant force F begins to act on a rock moving through deep space in the +x direction. Three possible functions of its motion are: (1) x = 4t - 3 (2) x = -4t² + 6t - 3 (3) x = 4t² + 6t - 3

(a) For time t > 0, which are possible functions x(t) for the rock's position?

(b) For which function is F directed opposite the rock's initial direction of motion?