【OG24-616题】The author of the passage mentions "escape velocity" in order to help explain which of the following?_mofa留学圈mofaGRE在线题库系统_GRE在线搜题_GRE题库解析_GRE真题机经_GRE课程_GRE培训机构

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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 author of the passage mentions "escape velocity" in order to help explain which of the following?

The tendency for asteroids to become smaller rather than larger over time The speed with which impact fragments reassemble when they do not escape an asteroid's gravitational attraction after a collision The frequency with which collisions among asteroids occur The rotation rates of asteroids smaller than 200 meters in diameter The tendency for large asteroids to persist after collisions

正确答案： E

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文章大意逻辑
文章主要探讨小行星的结构。首先提出假设，如果小行星是单体岩石（monoliths），其旋转速率图应呈钟形分布；若为碎石堆（rubble piles），则极快速旋转的尾部会缺失。接着，研究发现除5颗外，直径大于200米的小行星旋转速率有严格限制，由此得出直径大于200米的小行星是多组件结构或碎石堆的结论，这与计算机模拟相符。然后解释了碰撞时，大的小行星被撞碎后，碎片速度通常小于逃逸速度，重力会使其重新聚合成碎石堆；而小的小行星多为单体岩石，因为其撞击碎片易逃离微弱重力。 文章类型及逻辑简图
文章类型为现象解释型。逻辑简图： 1. Hypothesis about rotation rate distribution if asteroids are monoliths or rubble piles 2. Research finding: limit on rotation rate for asteroids larger than 200 meters 3. Conclusion: large asteroids are rubble piles, consistent with computer modeling 4. Explanation of why large asteroids form rubble piles and small asteroids are monolithic 做题方法及问题类型
这是一道细节目的题，通过定位文中“escape velocity”所在句子及其上下文来分析其作用。 选项分析
A选项：文中未提及小行星随时间变小而非变大的趋势与逃逸速度有关，排除。 B选项：文中没有讨论撞击碎片在未逃离小行星引力时重新组合的速度和逃逸速度的关系，排除。 C选项：文中没有提及小行星碰撞的频率和逃逸速度的关联，排除。 D选项：文中没有表明直径小于200米的小行星的旋转速率和逃逸速度有联系，排除。 E选项：根据文章，大的小行星碰撞后碎片速度小于逃逸速度，重力使其重新聚合，这解释了大的小行星在碰撞后能够存续的趋势，该项正确。

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