问题 选择题

如图所示,某轴承厂有一条滚珠传送带,传送带与水平面间的夹角为θ,上方A处有一滚珠送料口,欲使滚珠从送料口沿无摩擦的斜槽最快地送到传送带上,应采取的方法是(  )

A.沿AB所在的竖直方向安放斜槽

B.过A点向传送带做垂线,得垂足C,应沿AC方向安放斜槽

C.考虑路程和加速度两方面的因素,应在AB和AC之间某一适当位置安放斜槽

D.上述三种方法,滚珠滑到传送带上所用的时间相同

答案

物体沿着位于同一竖直圆上所有光滑细杆由静止下滑,到达圆周最低点的时间相等,所有无论θ多大,t是不变的,证明如下:

由几何关系可知lAC=2Rsinα

物体从A运动到C的过程中加速度a=gsinα

根据匀加速运动位移时间公式得:2Rsinα=

1
2
gsinαt2

解得:t=

4R
g
以题目中的AB为直径画圆,则必定过C点,如图所示,则从A点沿斜槽到达圆弧的时间相等,

所以沿AB和沿AC方向安放斜槽,时间相等,应在AB和AC之间的某一适当位置安放斜槽,时间最短,故C正确

故选:C

选择题
单项选择题

Few beyond California’ s technology crowd recognise the name Larry Sonsini; none within its circle could fail to. For four decades he has been lawyer, adviser and friend to many prominent companies and investors. Some consider him the most powerful person in Silicon Valley. Companies beg for his law firm to represent them. The 65-year-old chairman of Wilson Sonsini Goodrich & Rosati (WSG&R) has suddenly been thrust into the spotlight: first, for counselling many of the technology firms now under scrutiny for backdating stock options (and in some cases, serving on their boards); and more recently, as outside counsel to Hewlett-Packard (HP), for initially defending the board’s dubious investigative practices.

WSG&R boasts 600 lawyers and represents around half of Silicon Valley’s public companies, including Apple Sun Microsystems and Google. Last year it ranked first in private-equity and venture-capital deals, with nearly twice as many as its closest rival. Over the past five years WSG&R has worked on over 1 000 mergers and acquisitions, collectively worth over $ 260 billion.

The recent troubles cast a shadow over WSG&R’s reputation. Although Mr. Sonsini is not accused of wrongdoing himself, many of his firm’s clients are on the ropes. Former executives at Brocade Communications suffered criminal charges in July, Mr. Sonsini Served on Brocade’s board until last year and his firm was its outside counsel. He also was on the boards of Pixar, Echelon, Lattice Semiconductor, LSI Logic and Novell all firms at which the issuing of stock options is being called into question.

WSG&R dismisses the idea that Mr. Sonsini faced a conflict of interest by acting as both director and legal adviser to so many firms and says he did not advise HP in its investigation of board members. Mr. Sonsini initially said it was "well done and within legal limits". It now seems it was neither.

The statistics in the text indicate ()

A. the fame of WSG&R

B. the notoriety of Larry Sonsini

C. the boom of WSG&R

D. the obligation of Hewlett-Packard