问题 多选题

如图所示,可视为质点、质量为m的小球,在半径为R的竖直放置的光滑圆形管道内做圆周运动,下列有关说法中正确的是(  )

A.小球通过最高点的最小速度为

gR

B.小球通过最高点的最小速度为0

C.如果小球在最高点时的速度大小为2

gR
,则此时小球给管道壁有坚直向下作用力

D.如果小球在最低点时的速度大小为

5gR,则此时小球给管道间的压力大小为6mg

答案

A、B、圆形管道内能支撑小球,小球能够通过最高点时的最小速度为0,故A错误,B正确;

C、设管道对小球的弹力大小为F,方向竖直向下.由牛顿第二定律得

mg+F=m

v2
R
,v=2
gR
,代入解得F=3mg>0,方向竖直向下.

根据牛顿第三定律得知:小球对管道的弹力方向竖直向上,即小球对管道的外壁有作用力.

故C错误;

D、重力和支持力的合力提供向心力,根据牛顿第二定律,有:N-mg=m

v2
R

解得:N=mg+m

v2
r
=mg+m
5gR
R
=6mg;

根据牛顿第三定律,球对管道的压力大小为6mg;

故D正确;

故选BD.

阅读理解

阅读理解。

     Much has been written about body language during a job interview. Experts from across many fields

are full of advice on just how to present your best performance in an interview using conscious (自觉的)

body language choices. While many of these experts say there is a "secret" system for making the most of

an interview, the reality is that most of them focus on a few general types of behaviors that have the

greatest effect during a job interview.

     Entering the interview location - Your walk should be active and confident. Keep your shoulders

straight, head up, and make eye contact with the people around you. Maintain (保持) an active facial

expression and be confident when shaking hands and greeting people to whom you are introduced.

     During the interview - Sit in a straight and attentive posture. Never let your shoulders drop or your head hang down, but rather keep your head up and make eye contact with the interviewers. Don't play with any papers or notes you have in front of you, and don't lean (倚靠) your arms heavily on the table. Keep your body position open and make eye contact while listening to and answering questions.

     Leaving the interview location - No matter how you feel about your performance during the interview,

carry yourself with confidence till you leave the interview location. Be confident when shaking hands and

making eye contact as you say goodbye, and keep your body posture straight as you walk all the way to

your car.

     Remember, the more you make active and confident body language a part of your daily life, the more it becomes a habit that follows you wherever you go.

1. We can learn from the text that the "secret" system mainly refers to      .

A. body language

B. habits

C. language skills

D. advice

2. A proper posture in an interview is to      .

A. keep your head up                  

B. sit in a relaxed way

C. hold a piece of paper                

D. lean your arms on the table

3. During the interview, which of the following is NOT mentioned?

A. Eye contact.                 

B. Clothing style.

C. Sitting posture.

D. Facial expression.

4. What's the purpose of the text?

A. To show the general steps of an interview.

B. To explain the meaning of body language.

C. To persuade people to use more body language.

D. To give advice on how to carry yourself in an interview.

5. It can be inferred from the text that      .

A. knowledge plays a key role in an interview

B. active body language helps a lot in an interview

C. advice from experts makes a good result in an interview

D. communication skills are necessary for a good interview

填空题