Danny was just tired about the way things were going. His mum came to school and talked on and on about Rick Jackson. It seemed that she would never stop talking. “Somebody’s got to stop that boy!” she was shouting. “Rick’s troubling everybody in the neighborhood. And he loves to pick on little boys like Danny.”
Mrs. Green, Danny’s teacher, was concerned a lot. “I didn’t know that Danny was being picked on,” she said. “He’s never said anything about this to me!” Mrs. Green looked at Danny. “How long had this been going on?” she asked. Danny could only shake his head and look at the floor. He knew if he said a word about this, he would have trouble at school.
Danny hadn’t said anything about the problem because he wanted to do things with the boys in the neighborhood. After all, most of them were nice to him. He hated to leave the gang just because of Rick. Maybe the time had come to find new friends. He felt it hard to make up his mind.
角，细线的拉力为F；若用一力F2水平向左拉小车，使小球和车一起以加速度
向左运动时，细线与竖直方向也成
。则