[填空题]
You stand at the top of a cliff while your friend stands on rzc+p. bfd e 2 wg5;apo6z,917k gbcvtthe s22hhf0 3g,o, y-ah* zsy pmobground below you. You drop a ball from rest and see that it takes 2.8 s for the ball to hit the ground below. Your friend then picks up the ball and throwfs2o z*-bm2ayp ,0ogsh3yhh, s it up to you, such that it just comes to rest in your hand. What is the speed with which your friend threw the ball? $m/s$
