问题 计算题

(18分)、如图所示,一个冲击摆,它可以用来测量高速运动的子弹的速率。一质量m=10g的子弹,以一定水平速度射入冲击摆的木质摆锤中,冲击摆的摆锤上升至最大高度时摆线与竖直方向的夹角。设摆锤质量M=1kg,摆长L=0.9m,运动过程不计空气阻力,重力加速度

求:(1)子弹击中摆锤后两者的共同速度是多大?

(2)子弹的初速度是多大?

答案

题目分析:由题意,运动过程不计空气。

阻力摆锤和子弹一起摆动上升的过程机械能守恒,(文字说明2分)

则有:        ①(4分)

代入数据,解得:     ② (3分)

以子弹和摆锤为系统,击打过程内力远远大于外力作用

系统在水平方向上动量守恒,(文字说明2分)则有:   ③(4分)

代入数据,解得:      ④(3分)

单项选择题
单项选择题

It was a fixing sight: there, in the Capitol itself, a U.S. Senator often mocked for his halting, inarticulate speaking, reached deep into his Midwestern roots and spoke eloquently, even poetically, about who he was and what he believed, stunning politicians and journalists alike.

I refer, of course, to Senator Jefferson Smith. In Frank Capra’s classic Mr. Smith Goes to Washington, Jimmy Stewart plays this simple, idealistic small-town American, mocked and scorned by the big-moneyed, oh-so-sophisticated power elite--only to triumph over a corrupt Establishment with his rock-solid goodness.

At root, it is this role that soon-to-be-ex-Senator Bob Dole most aspires to play: the self effacing, quietly powerful small-town man from Main Street who outwits the cosmopolitan, slick-talking snob from the fleshpots. And why not There is, after all, no more enduring American icon.

How enduring Before Americans had a Constitution, Thomas Jefferson was arguing that the new nation’s future would depend on a base of agrarian yeomen free from the vices inherent in big cities. In 1840 one of the classic, image-driven presidential campaigns featured William Henry Harrison as the embodiment of rural virtues, the candidate of the log cabin and hard cider, defeating the incumbent Martin Van Buren, who was accused of dandified dress and manners.

There is, of course, a huge disconnect between this professed love of the simple, unspoiled life and the way Americans actually live. As a people, Americans have spent the better part of the 20th century deserting the farms and the small towns for the cities and the suburbs; and are torn between vacationing in Disney World and Las Vegas.

U.S. politicians too haven’t exactly shunned the temptations of the cosmopolitan life. The town of Russell, Kansas, often seems to be Dole’s running mate, but the candidate spends his leisure time in a luxury condominium in Bal Harbor, Florida. Bill Clinton still believes in a place called Hope, but the spiffy, celebrity-dense resorts of Martha’s Vineyard and Jackson Hole are where he kicks back. Ronald Reagan embodied the faith-and-family pieties of the front porch and Main Street, but he fled Iowa for a career and a life in Hollywood.

Still, the hunger for the way Americans believe they are supposed to live is p, and the distrust of the intellectual hustler with his airs and his high-flown language runs deep. It makes sense for the Dole campaign to make this a contest between Dole as the laconic, quiet man whose words can be trusted and Bill Clinton as the traveling salesman with a line of smooth patter but a suitcase full of damaged goods. It makes sense for Dole to make his campaign song Thank God I’m a Country Boy--even if he is humming it 9,200 m up in a corporate jet on his way to a Florida condo.

It seems that the author is most critical of some politicians’()

A. hypocrisy

B. elegance

C. prestige

D. absurdity