Consider a ball in a plane given an initial velocity u = (ux, uy) and then subject to the gravitational force -mgey. We denote the position of the ball by r(t) = x(t)ex + y(t)ey. Assume that r(0) = 0. (3a) Explain why the motion satisfies the system of differential equations a" = 0 and y" = -g. [2] (3b) Solve the system in (3a). [2]