题目

Will human beings find intelligent alien (外星的) life anytime soon? Probably not, according to theoretical physicist Stephen Hawking.Hawking made the prediction during the Breakthrough Starshot announcement in New York City on April 12. At the news conference, Hawking, along with Russian billionaire sponsor Yuri Milner and a group of scientists, detailed a new project that aims to send a large number of tiny, wafer-size (薄饼大小的) spaceships into space to the neighboring star system Alpha Centauri (半人马座阿尔法星).If these tiny spaceships travel at 20 percent of the speed 如图（a）所示，在光滑水平面上用恒力F拉质量1kg的单匝均匀正方形铜线框，在1位置以速度v0＝2m/s进入匀强磁场时开始计时t＝0，此时线框中感应电动势0.8V，在t＝3s时刻线框到达2位置开始离开匀强磁场。此过程中v-t图象如图（b）所示，那么恒力F的大小为_______ N，线框完全离开磁场的瞬间位置3速度为________ m/s。