题目

动词填空 阅读下面短文，用括号内所给动词的适当形式填空（必要时可加情态动词或助动词） There was a boy who was forced to a boarding(寄宿的)school by his parents. Before being sent away, this boy used to be the best student in his class. He was at the top in every competition. But things (1)______(change) after he went to the new school. His grades began(2)_______(drop) . He hated being in a group. He was lonely all the time. He felt he was worthless and that no one loved him.   His parents began to worry about  him. But even they did not k在建筑工地为了把地基砸实，经常要几个人用较重的石块或铁块来夯地．现在有一个质量为m的圆柱形铁块放在地面上，A、B、C为铁块上表面边缘平分圆周的三点，如图甲所示．现将三根绳子分别系在A、B、C三点，三个工人分别拉住绳子的另一端，每根绳子拉直时都与地面成30°角，俯视图如图乙所示．每个工人用相同的力拉绳子，要想使铁块离开地面，每根绳子上的拉力至少应为23mg23mg．