问题 选择题

滑块m以一定的初速度沿放在地面上的粗糙斜面体M向上滑,然后又返回,M未发生滑动,在此过程中,M对地面的摩擦力(  )

A.大小方向均未发生变化

B.大小,方向均变化

C.大小变化,方向未变

D.大小未变,方向变化

答案

物体先减速上滑,后加速下滑,加速度一直沿斜面向下,对整体受力分析,受到总重力、支持力和向左的静摩擦力,根据牛顿第二定律,有

x分析:f=masinθ ①

y分析:(M+m)g-N=(M+m)acosθ ②

物体上滑时,受力如图,根据牛顿第二定律,有

mgsinθ+μmgcosθ=ma1

物体下滑时,受力如图,根据牛顿第二定律,有

mgsinθ-μmgcosθ=ma2

由①式,地面对斜面体的静摩擦力方向一直未变;

由③④两式,物体沿斜面向上滑动时,加速度较大;再由①式,物体沿斜面向上滑动时,地面与斜面体间的摩擦力较大;

故选C.

单项选择题
阅读理解

As soon as you meet, or even see, a person, you form an impression of him based on his posture, gestures and facial expressions. This is why it is important to make a good first impression on the person who will be interviewing you. Within the first 60 seconds of meeting you, the interviewer will have formed an impression about what type of person you are and what kind of employee you would become, and 55% of this will be based only on your body language. Here are some tips to make that impression a good one.

First of all, relax. Think positive thoughts and you are more likely to come across as a positive, confident person during the interview. Dress smartly, as your appearance is one of the first things an interviewer will notice you. Stand up straight, and walk in with your shoulders back and your head up. Try to avoid slowing down as you walk into the room because this suggests fear and uncertainty. Shake hands with the interviewer firmly. You should have a positive handshake and a warm smile. Opening your coat or jacket as you sit down shows that you, too, are open.

Posture is important. You should be sitting well back in your seat. Sit up straight and lean forward slightly, but not too much. Sitting at too much of an angle (角度) expresses discomfort and distrust. Tow of the most common defensive (防卫的) signals are crossing your arms and legs, so try to avoid them.

Another important issue is eye contact. Looking someone in the eye shows confidence in yourself and trust in the other person. However, don’t overdo it, as too much eye contact or staring will make the other person uncomfortable. Use more eye contact when listening than when talking, and when you look away, look down. Looking up at the ceiling will make you seem bored or rude.

小题1:The purpose of the first paragraph is to tell readers      .

A.what a good first impression is

B.how to make a good first impression

C.how long it takes to make a first impression

D.the importance of making a good first impression小题2:Which of the following does the author advise you to do when you are being interviewed?

A.Cross your arms or legs.

B.Open your coat or jacket when sitting down.

C.Go into the room more slowly than usual.

D.Avoid looking at the eyes of the interviewer directly.小题3:If you lean forward too much, it means      .

A.you are very bored

B.you are too nervous

C.you don’t have confidence

D.you don’t trust others小题4:What is the best title for the text?

A.How to use body language to win an interview

B.How to answer questions in an interview

C.How to understand body language

D.How to deal with an interviewer