题目

已知两个共点力的合力为50N，分力F1的方向与合力F的方向成30°角，分力F2的大小为30N则（ ）A.F1的大小是唯一的 B.F2的方向是唯一的C.F2有两个可能的方向 D.F2可取任意方向 答案：【答案】C【解析】如图所示，分力F1的方向与合力F的方向成30°角，当分力F2与F1垂直时，分力F2有最小值，最小值为：所以F2有两个可能的方向，C正确，ABD错误。故选C。 At Harton College—an English boarding school for boys —there are many rules . 15-year-old Bob Sanders often breaks them . The boys can go into the town in the afternoon after class . But they must return to the school at six o’clock . One afternoon Bob walked to the town . He looked at the shops and then went to the cinema . After the film , he looked at his watch . It was after eight o’clock . He was a little worried . He walked back to Harton College as fast as possible . When he arrived , he ran quickly to the main entrance . It was locked . He went round the school building to another door , which was locked too . He looked up at the window of his dormitory on the third floor . The window was open . But it was quite dark , and he could not climb up the wall easily . Then he saw another open window on the ground floor . It was the window of the headmaster’s study . He looked into the room —no one was there . Bob quickly climbed on to the window sill（窗台）and jumped into the room . Just then he heard a noise . Then someone turned on a light in the corridor（走廊）. Bob looked around and then hid under the sofa . One minute later , Mr. Mannering , the headmaster , came in . He turned on the light on his desk , and sat down on the sofa . Then he opened a book and began to read . Bob lay under the sofa as quietly as possible . He couldn’t move . The floor was cold and uncomfortable . He looked at the headmaster’s shoes and socks for an hour . “Why doesn’t he get up and go to bed ?” Bob thought . Mr. Mannering read his book for another hour . Finally , the headmaster closed his book and stood up .He put the book on a shelf and walked towards the door . “Thanks heavens , he didn’t find me under the sofa ,” thought Bob . Then Mr. Mannering stopped and spoke towards the sofa . “ Would you turn off the light when you leave ?” He said , and left the study . 1．Bob didn’t go to his dormitory because______________. A．its door was locked B．the window was shut C．it was quite dark D．the wall was too high for him to climb up 2．Who had turned on a light in the corridor ? A．Bob himself        B．Another pupil     C．An office clerk    D．Mr. Mannering 3．When the headmaster came in , Bob______________. A．was sitting on the sofa B．was lying under the sofa C．hid himself behind the bookshelf D．hid himself under the sofa 4．It can be inferred that Mr . Mannering ______________. A．knew that Bob was still at the cinema B．knew that Bob was in the dormitory C．knew exactly where Bob was D．didn’t know clearly where Bob was