----------------------------- 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) + integral(v_y(t),t,0,t) p_x(t) = p_x(0) + integral(v_x(t),t,0,t) integral(v_y(t),t,0,t) = integral(v_0y + a_y * t) = v_0y + integral(a_y *t, 0, t) integral(xt,t) = 1/2 xt^2 integral(a_y t, 0, t) = 1/2 a_y t^2
