问题 计算题

如图所示,木板与水平地面间的夹角θ可以随意改变,当θ=30°时,可视为质点的一小木块恰好能沿着木板匀速下滑。若让该小木块从木板的底端以大小恒定的初速率v0的速度沿木板向上运动,随着θ的改变,小物块沿木板向上滑行的距离x将发生变化,重力加速度为g。

(1) 求小物块与木板间的动摩擦因数;

(2) 当θ角为何值时,小物块沿木板向上滑行的距离最小,并求出此最小值。

答案

(1)(2)

题目分析:(1)当θ=30°时,木块处于平衡状态,对木块受力分析:

沿斜面方向:……①       (2分)

垂直斜面方向 :……②      (2分)

解得,……③ (2分)

(2)当θ变化时,设沿斜面向上为正方向,木块的加速度为a,则:

……④       (2分)

木块的位移x为:……⑤       (2分)

       (1分)

上问计算得,则时x最小,

……  ⑥  (1分)

x最小值为……………. (2分)

阅读理解

阅读理解。

     Michael Jackson, the American pop legend, died of a cardiac arrest (心脏骤停) in a Los Angeles

hospital, just weeks before he hoped to resurrect (复活) his four-decade long career with a series of

sold-out shows in London. The pop superstar was taken to the University of California at Los Angeles

medical centre last night, and doctors tried resuscitation (使苏醒,使复活) in the ambulance. He did not

regain consciousness and was reported dead about three hours later.

    "My brother, the legendary King of Pop, passed away on Thursday 25 June at 2:26pm," his brother

Jermaine said, "We believe he suffered a cardiac arrest at his home, however the cause of his death is

unknown until the results of the autopsy (验尸) are known. The personal physician who was with him

at the time attempted to resuscitated him."

    A spokesman for the UCLA medical centre said, "When he arrived at hospital at approximately

l:14pm, a team of doctors attempted to resuscitate him for a period of more than one hour, but they

were unsuccessful." Police said they were investigating, which is the standard procedure in such cases.

    Jackson's death brought a tragic end to a long decline from his peak in the 1980s when he was music's

greatest all-around performer, a uniter of black and white music who shattered the race barrier on MTV. His 1982 album Thriller, which included the blockbuster hits Beat It, Billie Jean and Thriller is still the

best-selling album of all time, with over 50m copies sold worldwide.

     The world famous entertainer had planned a series of 50 concerts in London from 12 July. Although

in the last two decades his reputation was destroyed by charges of child molestation (骚扰) and his

fantastic public behavior, all tickets were sold within hours, proving the King of Pop had enduring appeal.

1. Where did Michael Jackson die? 

A. At home in Los Angeles.    

B. In a Los Angeles hospital.

C. On the stage in London.      

D. In an ambulance to hospital.

2. What caused Michael Jackson to die according to Jermaine?   

A. Heart disease.              

B. It's unknown before the results of the autopsy.

C. Working too hard.          

D. His personal physician's improper treatment.

3. Why were the police involved in investigating the death of Michael Jackson?

A. Because they believed he was murdered.

B. Because it was standard procedure in such cases.

C. Because Michael Jackson died suddenly.

D. Because his brother was suspicious of the truth of his death.

4. It can be inferred that Michael Jackson was            

A. a King of Pop with good reputation.

B. a King of Pop still playing on the stage before death.

C. indeed a bad man with fantastic public behavior.

D. a popular King of Pop in spite of ill fame.

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