问题 选择题

如图所示,一带正电小球穿在一根绝缘的粗糙直杆上,杆与水平方向成θ角,整个空间存在着竖直向上的匀强电场和垂直于纸面向外的匀强磁场,小球沿杆向下运动,在A点时的动能为100 J,在C点时动能减为零,B为AC的中点。在运动过程中下列说法正确的是  (   )

A.小球在B点时的动能为50 J

B.小球电势能的增加量等于重力势能的减少量

C.小球在AB段克服摩擦力做的功与在BC段克服摩擦力做的功相等

D.到达C点后小球可能沿杆向上运动

答案

答案:D

分析:由于从a到c的过程中小球的动能减小,则运动速度减小,小球所受的洛伦兹力减小,导致滑动摩擦力减小,所以在下滑过程中,电场力、摩擦力做负功,重力做正功.

解答:解:A、ab段和bc段合外力不同,因此合外力做的功也不同,所以根据动能定理,动能的变化量不同,A项错误;

B、电势能增加是由电场力做功决定,而重力势能减小是由重力做功决定,由于动能与重力势能减少,转化小球的电势能,因此B项错误;

C、小球与杆之间的压力减小,摩擦力也在减小,而ab=bc,所以小球在ab段克服摩擦力做的功与在bc段克服摩擦力做的功不相等,故C项错;

D、小球运动到c点后,有可能静止,也有可能沿杆向上运动,故D项正确.

故选:D

点评:考查动能大小与速度大小关系,及速度大小与洛伦兹力大小,洛伦兹力与滑动摩擦力的关系,同时突出电场力做功与重力做功及摩擦力做功与能量的关系.

多项选择题
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In the US, people prefer waiting for a table to sitting with people they don’t know. If you are sitting at a table with people you don’t know, it is impolite to light up a cigarette without asking if it will trouble them.

At American restaurant and coffee shops you are usually served cold water before you order. You may find the bread and butter is free, and if you order coffee, you may get a free refill(再装满).

Most cities and towns have no rules about opening and closing time for stores or restaurants, though they usually do make rules for bars. Especially in large cities, stores may be open 24 hours a day.

Serving in restaurant is often large, too large for many people. If you can’t finish your meal but would like to enjoy the food later, ask your waitress or waiter for a “doggie bag”. It may have a picture of a dog on it, but everyone knows you’re taking the food for yourself. Supper and dinner are both words for the evening meal. Some people have “Sunday dinner”. This is an especially big noon meal.

Tips are not usually added to the check. They are not included in the price of the meal, either. A tip of about 15% of the price of a meal is expected and you should leave it on the table when you leave. In some restaurants, a check is brought on a plate and you put your money there. Then the waiter or waitress brings you your change.

小题1:If you are walking in New York, you will find that _____.

A.you can go to the bars any time you like

B.stores may be open for the customers a day

C.restaurants have some rules about foreigners

D.closing time is often decided by the managers小题2:When you are taking a “doggie bag” on the street, we know that _____.

A.you have bought some food for your dogs

B.you are taking the rest of food for your dogs

C.you will take the food home and enjoy it later

D.you have put a dog in the bag and take it home小题3:What can we learn from the last paragraph?

A.Tips belong to a part of the price of the meal.

B.Tips have become an essential part of the life.

C.Tips are important in the US for the restaurant.

D.You can provide the tips for the manager directly.