[填空题]
You stand at the top ofd(*kep6t-t p5 ,kjbp * sjn:qm a cliff while0) lp.jjnil g* your friend 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 ll)injp jg .*0threw the ball? $m/s$
