题目

如图所示，一个木块从斜面上匀速滑下，下列说法正确的是（ ）A.木块只受到重力和斜面的支持力B.下滑过程中，木块的重力势能转化为动能C.下滑过程中，木块受平衡力作用D.下滑过程中，木块的机械能保持不变 答案：【答案】C【解析】AC．木块从斜面匀速滑下，处于平衡状态，受力平衡，除了受到重力和斜面的支持力外，一定还受到斜面的摩擦力作用，A错误，C正确；B．下滑过程中，由于木块的速度不变，质量也不变，那么动能是不变的，B错误；D．下滑过程中，木块的动能是不变的，但是木块所处的高度在减小Living one’s life is really like driving on a highway. First, to make a good driving, the driver must carefully examine the   1   of his car, just as he cares for his health for a good life.   2   he enters into the highway, he must   3   certain rules for his own safety. It also can be said that he must obey certain rules of society when living as a decent （体面的） adult. For example, on the highway, he is   4   to keep a constant （不断的）  5  , which can be compared with his   7   activities through his life. Neither driving nor living must be taken at too   7   a speed nor at too slow a speed．If he drives too   8 , the police will give him a   9  , just as he will be arrested when he breaks the law.   10   too conservatively （保守地）, he will be called an out-of-date person   11   the others driving at a（an）   12   speed. When he wants to change his lanes, he must give careful   13   to every direction of his car. It is sometimes dangerous to change the way of life   14   he can be sure of completion （完成,结束） by doing so. He   15   not make any unnecessary changing of lanes which gives him only danger. Finally, he must   16   where he is now by recognizing some  17   appearing now and then. If he takes the wrong way, he must return to the right as soon as he can. But he may thus lose   18   and energy both on the road and in life. On the whole, driving on a highway and living one’s life are both hard work.   19   if he is careful and serious enough,   20   will provide him with much pleasure. 1．A．seats                     B．condition             C．petrol                  D．passengers 2．A．Once                    B．While                  C．For                     D．Although 3．A．listen to                 B．find                    C．follow                 D．insist on 4．A．ordered                 B．supposed             C．believed               D．hoped 5．A．state                     B．speed                  C．mind                   D．level 6．A．outdoor                 B．broken                C．good                   D．continuous 7．A．large                     B．high                    C．limited                 D．expected 8．A．speedily                B．slowly                 C．hurriedly             D．willingly 9．A．ticket                    B．lift                      C．chance                D．hand 10．A．Acting                 B．Running              C．Getting                D．Working 11．A．persuading           B．getting                 C．bothering             D．refusing 12．A．average               B．normal                C．usual                   D．fast 13．A．notice                 B．attention              C．signs                   D．regards 14．A．if                        B．when                  C．unless                 D．because 15．A．dare                    B．need                    C．may                    D．should 16．A．think                   B．discover              C．prove                  D．seem 17．A．signs                   B．signals                 C．posters                D．symbols 18．A．money                B．luck                    C．time                    D．trust 19．A．Therefore            B．But                     C．And                    D．Then 20．A．it                        B．he                       C．they                   D．you