问题 计算题

如图所示,MN是一段在竖直平面内半径为1m的光滑的1/4圆弧轨道,轨道上存在水平向右的匀强电场。轨道的右侧有一垂直纸面向内的匀强磁场,磁感应强度为B1=0.1T。现有一带电量为+1C质量为100g的带电小球从M点由静止开始自由下滑,恰能沿NP方向做直线运动,并进入右侧的复合场。(NP沿复合场的中心线) 已知AB板间的电压为UBA=2V,板间距离d=2m,板的长度L=3m,若小球恰能从板的边沿飞出,NP沿复合场的中心线,g取10m/s2。试求:

(1)小球运动到N点时的速度v;

(2)水平向右的匀强电场电场强度E;

(3)复合场中的匀强磁场的磁感应强度B2

答案

解:(1)小球沿NP做直线运动,由平衡条件:mg=qvB1

得v=10 m/s

(2)小球从M到N的过程中,由动能定理:mgR+qER=

带入数据得:E=4N/C

(3)在板间复合场小球受电场力qU/d=1N/C与重力平衡,故小球做匀速圆周运动设运动半径为R,由几何知识:R2=L2+(R-d/2)2

得R=5m

由qvB2=mv2/R

得:B2=0.2T

阅读理解

阅读理解。

     Everyone knows that exercise is important. We all need to exercise. Doctors say it is good for us. It

makes your heart and body strong. Children who often exercise are more alert (灵活). That means they

do better in tests and schoolwork than those who don't exercise. 

     They are many ways to exercise. You can walk, run, swim, skate, or play ball games. Make sure you

exercise in the following ways: You have to like what you are doing. Exercise enough (足够的)  but not

too much. It's best to exercise twice a week. Thirty minutes (分钟) each time is enough. Try all kinds of

sports until (直到) you find one, two or even three sports that feel right for you. 

     You can exercise at fitness centers. They have a lot of equipment (设备) there. The equipment will

help exercise your arms, legs and other parts of your body to make you healthy. Some people buy

equipment for their homes. But it's very expensive.

1. the underline word "fitness centers" in the passage means _____. 

A. 商务中心  

B. 健身中心  

C. 医疗中心    

D. 购物中心

2. The sport "_____" is not told in the passage. 

A. playing soccer

B. skateboarding  

C. walking    

D. running

3. The doctors say that exercise can make your heart and body _____.

A. ill  

B. happy  

C. strong    

D. health

4. About "exercise", which statement of the following is NOT right? 

A. Thirty minutes' exercise each time is enough. 

B. You have to exercise too much every day. 

C. You needn't do all kinds of sports to keep healthy. 

D. You have to like the sports that you are doing.

5. It's best to exercise _____. 

A. at fitness centers  

B. in groups 

C. in the morning  

D. twice a week

单项选择题