请学过高中物理运动学的同学跟我做一道物理题

【背景资料】
我们假设有一台一汽解放J5P自卸货车，货车信息见此：http://is.gd/jzNLF。之所以要假定有这样一台车，是因为根据这里(http://is.gd/jzOaA)以及这里(http://is.gd/jzOnb) 提供的车头照片，我需要找一台与之一模一样的工程货车，很遗憾，我在一汽官网没有找到完全一样的，但找到一个相似度很高的就是J5P，需要说明的是，因为必须是工程货车，所以我不能在一汽官网后面的小型货车里找相似车。另外如果我这个车型有出入的话估计问题不大，因为与之相似的好几款车有一个关键数据都是一样的，我在后面会说。这是必要说明。

现在请大家找到一起官网我说的那台货车，找到表格里“性能参数”一项，里面有一项是制动距离 (满 载、车速60km/h)，这个数据是≤36.7(m)。需要说明的是，这些货车的制动距离都是这个数据，所以我上面说型号问题应该不大。为了方便起见，我们下面将取用36m(满载,60km/h)这个数据。

根据某事件新闻发布会的说法(http://is.gd/jzQhF)，上面某辆运送石料的工程车刹车距离是5.4m。

以上三段提供一些背景数据，现在我们再提供计算公式。我们知道，紧急刹车过程我们可以近似的看做一个加速度为负值的匀加速直线运动。适用公式[*](忽略正负号)：

v^2=2as　　　　[*]

这里，v^2是速度(即刹车前一刹那的车速)的平方，a是刹车制动加速度，s是制动距离。下面我们进行一些计算。

【问题一】一辆满载的J5P货车，在车速60km/h时制动距离36m，那么在制动距离5.4m的时候，其刹车前车速是多少？

【解答】根据公式[*]，在此情况下因为刹车动力系统不变，车辆本身质量不变，故认为制动加速度a不变，我们根据速度平方与制动距离成正比，可以计算得知

v(min)=sqrt[(60^2)*5.4/36]≈23.2(km/h)

【问题二】现在假设这辆车制动距离5.4m的时候是空载，那么其刹车前车速是多少？

【解答】这个问题稍微复杂一些，我们要分步来算。首先，我们需要查找上述汽车资料，得知该车空载时质量约为15.5t，满载时质量约为31t。由公式[*]以及公式F=ma，我们知道，在刹车系统以及轮胎不变的情况下我们可认为刹车时提供的阻力不变，再加上刹车前车速60km/h不变，可以得知，在空载情况下，刹车的制动距离正比于车辆质量，即，空载时，一辆60km/h的J5P货车制动距离大约是18m，即满载时的一半。

接下来我们知道，这辆空载的卡车的制动距离是5.4m，那么在此情形下，因为同一辆车同样空载情形下制动加速度可认为恒定，由公式[*]，可知这辆车刹车前的速度平方仍然正比于制动距离，那么计算得知
v(max)=sqrt[(60^2)*5.4/18]≈32.9(km/h)

【结论】
由以上计算得知，这辆刹车距离5.4m的卡车，根据其装载量的不同，刹车前车速大约处于23.2km/h到32.9km/h这个范围，因为无从得知当时车辆装载状况，只知道这是一辆装载石料的工程货车，所以如果已经运上了货物，肯定不可能是只装一点那么不经济的做法，则车速应该是接近最小值的，至少应该是小于30km/h。这辆拥有庞大体积的货车，可以在如此低的车速之下，把一个耳不聋眼不花的横穿马路的成年人撞倒，而且是以该成年人行走方向倒下而不是车的行进方向倒下。且不说撞倒之后可以碰巧让其躯体处于车底盘空隙处向上方弓起呈往后退缩状，而脖子正好处于车轮处，且不说身上只看到碾压看不到撞擦痕迹。我就给你们两个提示：

(1) F永远和a是一个方向你们总是知道的吧？哪个星球上会出现速度的改变与受力不在一个方向上的？是牛逼顿教你的还是马逼顿教你的？

(2) 这么低的车速这么大的车，这得多么风雨交加暗无天日的地方才能让一个感觉和运动器官正常(至少没有报道该死者是盲人或者聋哑人或者行动不便者)的成年人发现不了这么一个低速移动的庞然大物而无法躲避？

【补充说明】
1. 该物理题并不复杂，它的出现既不是因为蛋疼也不是因为娱乐，当然它也没法提供真相这种在贵国相当奢侈的东西。它只能提供一个很粗浅的看问题的视角。

2. 出题人，也就是我，只具备基本的物理知识，不具备汽车专业相关的知识，所以如果我在上文中有任何原理性的或者计算细节方面的错误，欢迎专业人士指出。

3. 您可以自由复制、传播、使用本文，甚至可以不用署我的名字和给出我的链接地址，只要您不是出于商业目的或者其他不可告人的目的。本文只负责计算，不负责煽动、灭口、提供证据等其他服务。

Beautiful English(3): When Love Beckons You

When Love Beckons You
Kahlil Gibran

When love beckons to you, follow him, though his ways are hard and steep. And when his wings enfold you, yield to him, though the sword hidden among his pinions may wound you. And when he speaks to himyou, believe in him, though his voice may shatter your dreams as the north winds lays waste the garden.

For even as love crowns you, so shall he crucify you. Even as he is for your growth, so is he for your pruning. Even as he ascends to your height and caresses your tenderest branches that quiver in the sun, so shall the descend to your roots and shake them in their clinging to the earth.

But if, in your fear, you would seek only love's peace and love's pleasure, then it is better for you that you cover your nakedness and pass out of love's threshing-floor, tointo the seasonless world where you shouldshall laugh, but not all of your laughter, and weep, but not all of your tears. Love gives naught but itself and takes naught but from itself. Love possesses not, nor would it be possessed, for love is sufficient unto love.

Love has no other desires but to fulfill itself. But if you love and must have desires, let these be your desires:

To melt and be like a running brook that sings its melody to the night.
To know the pain of too much tenderness.
To be wounded by your own understanding of love;
And to bleed willingly and joyfully.
To wake at dawn with a winged heart and give thanks for another day of loving;
To rest at the noon hour and meditate love's ecstasy;
To return home at eventide with gratitude;
And then to sleep with a prayer for the beloved in your heart and a song of praise upon your lips.

Beautiful English(2): What I have Lived for

What I have Lived for

Bertrand Russell

Three passions, simple but overwhelmingly strong, have governed my life: the longing for love, the search for knowledge, and the unbearable pity for the suffering of mankind. These passions, like great winds, have blown me hither and thither, in a wayward course, over a deep ocean of anguish, reaching to the very verge of dispair.

I have sought love, first, because it brings ecstasy － ecstasy so great that I would often have sacrificed all the rest of my life for a few hours for this joy. I have sought it, next, because it releaves[relieves] loneliness － that terrible loneliness in which one shivering conciousness[consciousness] looks over the rim [of] the world into the cold unfathomable lifeless abyss. I have sought it, finally, because in the union of love I have seen, in a mystic miniature, the prefiguring vision of the heaven that saints and poets have imagined. This is what I sought, [and] though it might seem too good for human life, this is what - at last - I have found.

With equal passion I have sought knowledge. I have wished to understand the hearts of men. I have wished to know why stars shine. And I have tried to apprehend the Pythagorean power by which number holds sway above the flux. A little of this, but not much, I have achieved.

Beautiful English(1): Gettysburg Address

Gettysburg Address
Abraham Lincoln

Four score[Fourscore] and seven years ago, our fathers brought forth upon this continent a new nation, conceived in liberty, and dedicated to the proposition that all men are created equal.

Now, we are engaged in a great civil war, testing whether that nation or any nation so conceived and so dedicated, can long endure. We are met on a great battlefeild of that war. We have come to dedicate a portion of that field as a final resting-place for those who here gave their lives that that nation might live. It is altogether fitting and proper that we should do this.

But, in a larger sense, we cannot dedicate, we cannot consecrate, we cannot hallow this ground. The brave men, living and dead, [Who struggled here,] have consecrated it far above our poor power to add or detract. The world will little note nor long remember what we say here, but it can never forget what they did here. It is for us, the living, rather, to be dedicated here to the unfinished work which they who fought here have thus far so nobly advanced. It is rather for us to be here dedicated to the great task remaining before us － that from these honored dead we take increased devotion to that cause [for] which they gave the last full measure of devotion; that we here highly resolve that these dead shall not have died in vain; that this nation, under God, shall have a new birth of freedom; and that government of the people, by the people, and for the people, shall not perish from the earth.

Report the Score of GRE General Test

Though it is not very good, I am still satisfied. For I didn't make enough effort to prepare this test, I am content with this score.

Cannot see the score? Find means by youself. Haha...

Some pictures of D. melanogaster from Wikipedia

male of wild type

I guess they are three pictures of a single fruitfly from different perspectives. Yet I cannot provide sufficient evidences.

I think the lateral one is the most beautiful. If I denominated the fruitfly, I would name it "Drosophila elegans". Hahaha...

By the way, it is so difficult for me to distinguish the mutant character of the fruitfly which we call it bar eyes and superbar eyes. I need more practices.

Wiki...

Be blockaded again.

I come back

Crush the blockade of goddamn gfw.
Vive la Liberté!

The Beautiful World Inside The Cell

http://aimediaserver.com/studiodaily/harvard/harvard.swf

An excellent animation about how the inner world of the cell functions. CERNET users should find a good proxy to appreciate this flash.