A ball is thrown from ground at an angle with horizontal and with an initial speed u 0 . For the resulting projectile motion, the magnitude of average velocity of the ball up to the point when it hits the ground for the first time is V 1 . After hitting the ground, the ball rebounds at the same angle but with a reduced speed of . Its motion continues for a long time as shown in figure. If the magnitude of average velocity of the ball for entire duration of motion is 0.8 V 1 , the value of is________.