题干

质量为2kg的物体A放在水平地面上,利用如图所示的装置使物体A以0.1m/s的速度做匀速直线运动,拉力F始终保持水平,弹簧测力计的示数为6N,若地面对物体的摩擦力为9N,不计弹簧测力计、滑轮及绳重,下面判断正确的是(   )

A:绳子自由端移动的速度为0.2m/s

B:拉力的功率为1.2W

C:2s内克服地面对物体摩擦力做功3.6J

D:滑轮组的机械效率为75%

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答案(点此获取答案解析)

A,B,D

同类题1

任务型阅读

    Here are some easy steps to create better habits that will surely improve your mental health.

    Get enough sleep. Keeping a consistent sleep schedule will ensure that your body and mind will function properly. Experts recommend that each person should sleep for at least 6. 5 to 8 hours a night____When you restart, it refreshes everything and helps your machine function faster and better.

    DO some exercise. You should know that regular exercise is necessary for a healthy body and soul. Some workout motivation tips from experts are that you should determine what exercises interest you and work your way up from there. Still no idea how to start?____

    Mind what you eat.____You know your body and you also know potato chips have done nothing except make you feel sluggish and guilty. DO your heart and waistline a favor by feeding your body healthy thing like lean protein, whole grains and fiber.

    ____ Use journaling as a way to release(释放) tension you may be holding in. You can also use it as a way to organize your thoughts, transfer the chaos from head to paper. You don't necessarily have to write for an audience. In fact, writing for yourself can be regarded as self-treatment and help you become more aware of yourself and your own thoughts.

    Explore the world. You don't need your passport for this one____ No matter how old you are, there are many things in this life that you have yet to experience. Get out of your comfort zone and see what the world has to offer you. You may actually be pleasantly surprised to try things you have never done before and how enjoyable they are.

A. Start writing.

B. Balance yourself.

C. Put down the hamburger and accept whole grains!

D. Hire a personal trainer to help you get off your sofa.

E. X Think of your body as a computer that needs some rebooting(重启).

F. Go out to new restaurants or hiking trails to expand your experiences.

G. Even if you have to work out slow at first, it's better to start than not at all.

同类题5

阅读下面的文字,完成下面小题。

夜航船

董宏量

    ①对住在江畔河边的人来说,夜航船是一幅看厌了的风景。然而,我怎么也忘记不了儿时看夜航船所引起的心灵的颤动。如果你的窗口正对着墨汁般流淌的夜江,你就可以想象一个爱沉思的孩子,在关灯后的深夜趴在冰冷的窗台上看夜航船的情景。那熟睡的长江,那一声声甜鼾似的汽笛,那江中闪烁的流动着的灯火,就像一个个美丽的童话,一个个漂浮着的梦,不知给了我多少温暖的安慰和神秘的向往。

    ②长大以后,夜航船不再神秘,我已无数次在长江上夜行。在相当长的一段时间里,我是天天坐轮渡船上班下班的,常常驰行在破晓的夜色和渐浓的暮色中,借着船上昏黄的灯光看书,夜航的里程,是一页又一页书的里程。我在渡船上读了很多很多的书,直到读到一本书也叫《夜航船》,才又唤起我对夜航船的迷恋和沉思。

    ③那是明代大学者张岱的《夜航船》,是一部令人惊叹的古文化的百科全书。他在书中断言:天下学问,惟夜航船中最难对付。他讲述了一个书生和一个和尚的故事,在狭窄的船舱里,和尚曾因书生高谈阔论而畏慑,又因书生的无知而大胆伸脚。他以“夜航船”为书名,其旨意令人深思,也使我对那古老的吴山越水中驰行的夜航船产生了浓厚的兴趣。与长江上的铁轮相比,那木船桡声当然富有传统的诗意,展示着一种令人品味的文化陈迹。

    ④有一年秋天,我终于坐上了从杭州到苏州的夜航船。在古老的运河里,在深沉的夜色中,木船就像一个怀旧的老人,缓缓地追寻着发黄的陈迹。虽然这是机动船,哒哒哒的马达声会打破怀古的幽情,但你仍可通过那剪影似的小桥小镇,想起历代江南才子吟咏的诗词和故事。这条河流沉淀着太多的文思,使任何一只船都像一个温柔的手指,抚摸着一行诗、一行典丽的汉字。

    ⑤然而,我还是喜欢长江上的夜航船,这不仅仅是出于对家乡的偏爱。在无边的大海上夜行,你会有一种茫然的漂泊感,会产生“我从哪里来?我到哪里去?”的惶惑。在小河中夜行,你又会感到生命的逼仄和拘谨。只有在浩荡的长江上,你才会更美妙地体会到夜航的乐趣,有一种掌握着自己的命运又放胆漂泊的感觉。你知道你是顺水而下或逆流而上,两边的岸离你既近又远,你可穿行在高峡夹峙的一线激流中,也可飘荡在豁然开阔的万顷碧波里。当夜深无人你独自抚栏,看天上的星星与两岸时隐时现的灯火交融在一起,你还会想到,此时是不是有一个孩子趴在窗口,像当年的你一样看夜航船,看他的梦呢?

    ⑥当然,夜航船的最大好处还是读书。不论是短途还是长途的夜航船,你都会在宁静的漂流中情不自禁地捧起一本书来。在这没有颠簸的旅行中,在水流和夜色把你隔绝成一个岛的时光里,在悠闲的氛围中,你读书会比平时更加投入也更加愉悦。于是,你无形中就到达了某个港口,那不仅是船的到达,还有书带给你的某种感悟的到达。

    ⑦对于习惯静夜工作的人,“夜航船”是一种美丽的象征。我们在夜色中执著地航行着,写诗的笔、在音乐中起伏的指挥棒、对烈焰挥舞的钢钎,甚至清洁工的扫帚,都会使我们想起桨,划动时光之浪的桨。我们就这样不停地到达,也不停地出发,夜航在生命的河流上。于是夜就充满了创造,充满了活力,充满了遐思和梦想,使每一个看见夜航船灯火的人,都不再寂寞,不再孤独,都想追着那灯火而去,去渐渐明亮起来的远方……