题干

如图示意黄河干流沿河堤剖面及各河段干流长度,读图完成以下问题。

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同类题3

阅读下文,回答相关问题。

                                                                                 太空中的绿色动力——太阳帆
        ①太阳光传送光和热,照到人身上,人会感到暖洋洋的,但从来也没有人感觉到太阳光有压力。实际上,太阳光是有压力的,因为光具有两重性,既是电磁波,又是粒子——光子。光线实际上是光子流,当光子流受到物体阻挡时,光子就撞到该物体上,就像空气分子撞到物体上一样,它的动能就转化成对物体的压力。
      ②不过,太阳光产生的压力——光压是非常非常小的。不仅人感受不到,就连普通的仪器也测不出来。在地球附近,太阳光照射到一个平整、光亮、能完全反射光的表面时,产生的压力最大,大约是9×10 牛/平方米,也就是说100万平方米平整光亮的面积上才受到9牛的压力,只相当于一个2分硬币的重量。在地面上,由于重力、大气压力、空气阻力、摩擦力等力的存在,微乎其微的太阳光压力被淹没在这些宏观力的汪洋大海之中。
         ③“山中无老虎,猴子称大王。”到了太空中,重力、大气压力、空气阻力、摩擦力等几乎完全消失,太阳光压才有了出头之日。一些具有创新思维的人开始想到利用太阳光压来推动航天器在太空飞行。早在上一世纪初,俄罗斯宇航理论先驱齐奥尔科夫斯基就提出过这一大胆的设想。以后,又有不少科学家进行过研究。然而,只有当科学技术发展到今天的水平,在有强大的火箭把航天器送入太空的条件下,利用太阳光作为航天推进力才有了实现的可能。
        ④太阳光压的大小是与接受太阳照射的面积成正比的。受照面积越大,产生的压力越大。为了获得一定的压力,必须有足够大的受照面积,从而引出了太阳帆的概念。
        ⑤太阳帆是一种面积很大,表面平整、光滑、无斑点和皱纹的薄膜,一般由聚酯或聚酰亚胺等高分子材料制成,表面镀铝或银,使其具有全反射的特性。
        ⑥一块面积为105×105平方米的太阳帆,在太阳光正射下可获得大约100毫牛的力,用它推动100千克的物体,可产生1毫米/平方秒的加速度。这个加速度极其微小,只有地面重力加速度的万分之一。
      ⑦俗话说:“涓涓细流汇成大海,块块碎土堆成高山。”速度等于加速度与时间的乘积,尽管加速度非常小,只要时间足够长,终能达到一定的速度。即使航天器的加速度只有1毫米/平方秒,那么,一天以后,速度达到86.4米/秒;一个月后达到2592米/秒;130天后,就可超过第二宇宙速度,达到11.23千米/秒;一年后可达到31.54千米/秒,足以飞出太阳系,由此可见加速度不在大,时间长则灵。

同类题5

根据短文内容,从短文后的选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。

    Life is just like a box. Inside it are happiness and sorrow, failure and success, hope and despair. Life is a learning process. Experiences in it teach us new lessons and make us a better person.

    Love

    Love plays a key role in our life. Love makes you feel wanted. Without love a person could become cruel and violent. In the early stage of our life, our parents are the ones who show us with unconditional love and care. They tell us what is right or wrong, good or bad. ____.It is only after marriage and having kids that a person understands and becomes sensitive to the feelings of others. Kids make a person responsible and mature and help us to understand life better.

    Happiness and Sorrow

    Materialistic happiness is short­lived, but happiness achieved by bringing a smile on others gives a certain level of fulfillment.  ____.No mind is happy without peace. We realize the true worth of happiness when we are in sorrow. Sorrow is basically due to death of a loved one, failure and despair. But these things are temporary and pass away.

    Failure and Success

    ____.It helps us to touch the sky, teaches us to survive and shows us a specific way.

    Hope and Despair

    Hope is what keeps life going. Parents always hope their children will do well. Hope makes us dream. Hope builds in patience. ____, because after every night there is a day. Nothing remains the same. We have only one choice—keep moving on in life and be hopeful.

Life teaches us not to regret over yesterday, for it has passed and is beyond our control. Tomorrow is unknown, for it could either be right or dull. ____, so that we will enjoy a better tomorrow.

A. Life teaches us not to despair even in the darkest hour

B. So the only alternative is work hard today

C. But we always tend to take this for granted

D. Success lies in trusting yourself

E. So let's enjoy every day

F. Failure is the path to success

G. Peace of mind is the main link to happiness