问题 选择题

如图所示,固定的光滑斜面上,有一物体P放在物体Q上,二者一道以某一初速度沿斜面向上滑行,然后滑下.上行和下滑过程中两者始终保持相对静止.下列说法中正确的是(  )

A.上升过程中P受Q的摩擦力方向向上

B.下滑过程中P受Q的摩擦力方向向上

C.上升过程中P不受Q的摩擦力

D.下滑过程中P受Q的摩擦力方向向下

答案

在上升和下滑的过程,整体都是只受两个力,重力和支持力,根据牛顿第二定律得,a=

Mgsinθ
M
=gsinθ,方向沿斜面向下.

  设P所受的摩擦力向上,根据牛顿第二定律有:mgsinθ-f=ma,将a带入,求得f=0.知上升和下滑过程中P不受摩擦力.故C正确,A、B、D错误.

故选C.

单项选择题
阅读理解

阅读理解。

     A lot of us lose life's tough battles by starting a frontal attack-when a touch of humor might well enable

us to win. Consider the case of a young friend of mine, who hit a traffic jam on his way to work shortly

after receiving an ultimatum (最后通牒) about being late on the job. Although there was a good reason for

Sam's being late-serious illness at home-he decided that this by-now-familiar excuse wouldn't work any

longer. His supervisor was probably already pacing up and down preparing a dismissal speech.

     Yes, the boss was. Sam entered the office at 9:35. The place was as quiet as a locker room; everyone

was hard at work. Sam's supervisor came up to him. Suddenly, Sam forced a grin and stretched out his

hand. "How do you do!" he said. "I'm Sam Maynard. I'm applying for a job, which, I understand, became

available just 35 minutes ago. Does the early bird get the worm?"

     The room exploded in laughter. The supervisor clamped off a smile and walked back to his office. Sam

Maynard had saved his job-with the only tool that could win, a laugh.

     Humor is a most effective, yet frequently neglected, means of handling the difficult situations in our lives.

It can be used for patching up differences, apologizing, saying "no", criticizing, getting the other fellow to

do what you want without his losing face. For some jobs, it's the only tool that can succeed. It is a way to

discuss subjects so sensitive that serious dialog may start a quarrel. For example, many believe that comedians

on television are doing more today for racial and religious tolerance than people in any other forum.

1. Why was Sam late for his job?

A. Because he was ill.

B. Because he was caught in a traffic jam.

C. He was busy applying for a new job.

D. Because he got up late.

2. The phrase "clamped off" in Paragraph 3 means ____.

A. burst into

B. tried to set up

C. tried to hold back

D. gave out

3. Which of the following statements can we infer from the passage?

A. Sam was supposed to come to his office at 8:30.

B. It wasn't the first time that Sam came late for his work.

C. Humor is the most effective way of solving problems.

D. All people lose life's battles for they are lacking in a sense of humor.