[填空题]
A 1.0-kg trash-can lid is cqjr qrh 4 8kbr7k62x;suspended agaiz2tras :y1/q-etnqs1 nst gravity by tennis balls thrown vertically upward at it. How many tennis balls per second must rebound from the ltsntyqzar11qs e/ :2- id elastically, assuming they have a mass of 0.060 kg and are thrown at 12m/s balls/s