A wrecking ball swings at the end of a 13.9 meter cable on a vertical circular arc. The crane operator manages to give the ball a speed of 12.6 m/s as the ball passes through the lowest point of its swing and then gives the ball no further assistance. Air resistance and friction are negligible. Determine what speed vf does the ball have when the cable makes an angle of 25.9 degree with respect to the vertical?