----------------------------- parabola solving.
p(0) = x0,y0 = (0,0) a(t) = 0,-g v(0) = v0_x,v0_y v(t) = v(0) + a(t) * t v(t)_x = v0_x + a_x * t v(t)_y = v0_y + a_y * t p_y(t) = p_y(0) + v_y(0) * t + a_y(t) t^2 / 2 p_x(t) = p_x(0) + v_x(0) * t + a_x(t) t^2 / 2 p_y(t) = 0 + v_y(0) * t - g t^2 / 2 p_x(t) = 0 + v_x(0) * t
