如图所示，一个物体放在水平面上，在跟竖直方向成θ角的斜向下的推力F的作用下沿平面移动了距离s．若物体的质量为m，物体与地面之间的摩擦力大小为f，则在此过程中（ ）
A．摩擦力做的功为fs
B．力F做的功为Fscosθ
C．力F做的功为Fssinθ
D．重力做的功为MgS
A、物体与地面之间的摩擦力大小为f，物体的位移的大小为s，由功的公式可得W=-fs，所以A错误．
B、力F与竖直方向成θ角，所以在水平方向的分力为Fsinθ，所以F做的功为Fs•sinθ，所以B错误．
C、由B的分析可知C正确．
D、重力在竖直方向上，物体在竖直方向的位移是零，所以重力的功为零，所以D错误．
故选C．
The situation seemed hopeless. From the first day he entered my junior-high classroom, Willard P. Franklin lived in his own , shutting out his classmates and me, his teacher. I tried to build a relationship with him but he showed no interest. Even a “Good Morning Willar” received only a grunt(咕哝) that couldn’t be . Willard was a loner(孤独的人) who seemed to have no hope or need to break silence. Shortly after the Thanksgiving holiday, we received the news of the annual(每年的) Christmas collection of money for the less people in our school district. “Christmas is a season of ,” I told my students. “There are a few students in the school who might not have a happy holiday season. donating our Christmas collection, you will help buy food, clothing and toys for these needy people. We start the collection tomorrow.” When I called for the donations the next day, I discovered that everyone had forgotten, except Willard P. Franklin. The boy dug deep into his pants pockets he walked up to my desk. Carefully, he two coins into the small box. “I don’t need milk for lunch,” he said . For a moment, just a moment, he smiled . Then he turned and walked back to his desk. That night, after school, I took our to the school principal. I couldn’t sharing the incident(事件) that had taken place. “I may be wrong, but I believe Willard might be getting ready to become a part of the world around him,” I told the principal. “Yes, I believe it is for him to get out of his own world,” he nodded. “And I think we can do something to have him a bit of his world with us. I just received a of the poor families in our school who most need help through the Christmas collection. Here, take a look at it.” As I read, I discovered Willard P. Franklin and his family were the top names on the list.
|