OG阅读 天文篇，不会呀

GMAT要上700

When asteroids collide, some collisions cause an asteroid to spin faster; others slow it down. If asteroids are all monoliths-single rocks-undergoing random collisions, a graph of their rotation rates should show a bell-shaped distribution with statistical “tails” of very fast and very slow rotators. If asteroids are rubble piles, however, the tail representing the very fast rotators would be missing, because any loose aggregate spinning faster than once every few hours (depending on the asteroid’s bulk density) would fly apart. Researchers have discovered that all but five observed asteroids obey a strict limit on rate of rotation. The exceptions are all smaller than 200 meters in diameter, with an abrupt cutoff for asteroids larger than that.

♦The evident conclusion-that asteroids larger than♦ 200 meters across are multicomponent structures or rubble piles-agrees with recent computer modeling of collisions, which also finds a transition at that diameter. A collision can blast a large asteroid to bits, but after the collision those bits will usually move slower than their mutual escape velocity. Over several hours, gravity will reassemble all but the fastest pieces into a rubble pile. Because collisions among asteroids are relatively frequent, most large bodies have already suffered this fate. Conversely, most small asteroids should be monolithic, because impact fragments easily escape their feeble gravity.






The discovery of which of the following would call into question the conclusion mentioned in line 16 ?

• AAn asteroid 100 meters in diameter rotating at a rate of once per week
• BAn asteroid 150 meters in diameter rotating at a rate of 20 times per hour
• CAn asteroid 250 meters in diameter rotating at a rate of once per week
• DAn asteroid 500 meters in diameter rotating at a rate of once per hour
• EAn asteroid 1,000 meters in diameter rotating at a rate of once every 24 hours
• 怎么都想不明白为什么这道题选D，文章不是说那些直径大于200的rubble piles 转的更快，once a few hour。选D不就是增强而不是削弱了吗？
• . A collision can blast a large asteroid to bits, but after the collision those bits will usually move♦ slower than their mutual escape velocity. Over several♦ hours, gravity will reassemble all but the fastest pieces into a rubble pile. Because collisions among asteroids are relatively frequent, most large bodies have already suffered this fate.
• The author of the passage mentions "escape velocity" (see line 22) in order to help explain which of the following?
  
  • AThe tendency for asteroids to become smaller rather than larger over time
  • BThe speed with which impact fragments reassemble when they do not escape an asteroid’s gravitational attraction after a collision
  • CThe frequency with which collisions among asteroids occur
  • DThe rotation rates of asteroids smaller than 200 meters in diameter
  • EThe tendency for large asteroids to persist after collisions
  • 为什么B是错的 ？
    
  
  

奋斗小女子

第一道题在文章中的定位是：

If asteroids are rubble piles, however, the tail representing the very fast rotators would be missing, because any loose aggregate spinning faster than once every few hours (depending on the asteroid’s bulk density) would fly apart.

好，那么从文章中我们可以知道, 是rubble pile形态的asteroids体积都比较大。逻辑链是： if.... rubble piles...--> fast rotators would be mssing ---> because...不会faster than ONCE EVERY FEW HOURS.

那么重点就是找答案中大的asteroid，如果转快过once every few hours，则"would call into questions the conclusion"
C - 250meters大的， 一星期转一次   好慢啊 排除
D - 500meters大的， 一小时转一次， 比once every few hours快  （正确答案）
E - 1000meters大的， 24小时转一次， 比once every few hours慢， 符合文中内容。

奋斗小女子

第二道题目， 原文中定位：

A collision can blast a large asteroid to bits, but after the collision those bits will usually move♦ slower than their mutual escape velocity. Over several♦ hours, gravity will reassemble all but the fastest pieces into a rubble pile.

B错误的原因是 - 这里讲的是'escape velocity' - aka 'speed of escape' ， 目的并不是为了讲'speed of reassemble'  ， speed of reassemble属于一个新的概念文中并没有提到。

原文这里的内容主要是想讲， collision之后bits的escape velosity和它们后来reassemble成rubble pile之间的关系。

这一题真的挺难的。。。

GMAT要上700

奋斗小女子 发表于 2017-7-7 10:34
第一道题在文章中的定位是：

If asteroids are rubble piles, however, the tail representing the very  ...

你好~谢谢你！但我好像还是没有看懂，我理解成了rubble piles 都是直径大于200的，然后他们旋转的特别快，所以尾巴就在高速旋转中丢失了。然后要质疑的话应该就是质疑他们就是取反，说那些大直径的旋转的不是很快。那么就不应该选D了吧，因为那是增强了，直径大于200，还旋转的更快

GMAT要上700

奋斗小女子 发表于 2017-7-7 11:15
第二道题目， 原文中定位：

A collision can blast a large asteroid to bits, but after the collision t ...

谢谢小仙女

奋斗小女子

GMAT要上700 发表于 2017-7-13 13:18
谢谢小仙女

不客气~ 加油！

GMAT要上700

奋斗小女子 发表于 2017-7-13 13:50
不客气~ 加油！

小仙女，我上一道题还是不懂就是反驳的那一道
我理解成了rubble piles 都是直径大于200的，然后他们旋转的特别快，所以尾巴就在高速旋转中丢失了。然后要质疑的话应该就是质疑他们就是取反，说那些大直径的旋转的不是很快。那么就不应该选D了吧，因为那是增强了，直径大于200，还旋转的更快

奋斗小女子

GMAT要上700 发表于 2017-7-13 14:38
小仙女，我上一道题还是不懂就是反驳的那一道
我理解成了rubble piles 都是直径大于200的，然后他们旋 ...

哦~~我大概明白你的意思了。

还是理解上出现了偏差， “the tail representing the very fast rotators would be missing” 指的是在那个"a graph of their ratation rates' 上面， 不会有显示这些rubble piles转的很快的‘数据’。 那个tail不是指小型星的尾巴。

所以这里说的就是rubble pie转的“不快”， weaken它就要说 其实发现它转的快。

GMAT要上700

奋斗小女子 发表于 2017-7-14 09:54
哦~~我大概明白你的意思了。

还是理解上出现了偏差， “the tail representing the very fast rotators  ...

终于懂了，谢谢小仙女

dayuan918

奋斗小女子 发表于 2017-7-14 09:54
哦~~我大概明白你的意思了。

还是理解上出现了偏差， “the tail representing the very fast rotators  ...

同意！