[填空题]
You stand at the top of a cliff while your fri zdd6o0()* g *oat+ kudcf/oplenp4 ns6vv-nou+ d stands on the ground 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 throws it up to you, such that it just comes to rest in your hand. What is the speed with which your friend threw thn6 puv s4+v-one ball? $m/s$
