[填空题]
You stand at the top of a cliff while ugwd 8;uaz(:f (+c* ;bkvnccgyour friend stands on the ground below vg5mcqrv )n7rzu5g 0: 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 bmr5:)gvvqz 7r05 g uncall 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 the ball? $m/s$
