4.9. example. show that if the velocity potential of an irrotational fluid motion is =ycosê, where (r, 0, y) are the spherical polar co-ordinates of any r point

4.9. Example. Show that if the velocity potential of an irrotational fluid motion is =ycosê, where (r, 0, y) are the spherical polar co-ordinates of any r point, the lines of flow lie on the surface r =k sin-0, k being a constant.