问题 问答题

如图所示,两条足够长的平行金属导轨相距L,与水平面的夹角为q,整个空间存在垂直于导轨平面的匀强磁场,磁感应强度大小均为B,虚线上方轨道光滑且磁场方向向上,虚线下方轨道粗糙且磁场方向向下.当导体棒EF以初速度v0沿导轨上滑至最大高度的过程中,导体棒MN一直静止在导轨上,若两导体棒质量均为m、电阻均为R,导轨电阻不计,重力加速度为g,在此过程中导体棒EF上产生的焦耳热为Q,求:

(1)导体棒MN受到的最大摩擦力;

(2)导体棒EF上升的最大高度.

答案

(1)导体棒EF向上做减速运动,产生的感应电动势和感应电流逐渐减小,MN所受的安培力方向沿导轨向下,大小不断减小,所以EF棒刚开始运动时MN所受的摩擦力最大.

EF获得向上初速度v0时,产生感应电动势 E=BLv0

电路中电流为I,由闭合电路欧姆定律:I=

E
2R

此时对导体棒MN受力分析,由平衡条件:FA+mgsinθ=f ③

FA=BIL ④

解得:f=

B2L2v0
2R
+mgsinθ ⑤

(2)导体棒上升过程MN一直静止,对系统由能的转化和守恒定律得:

1
2
m
v20
=mgh+2Q ⑥

解得:h=

m
v20
-4Q
2mg

答:(1)导体棒MN受到的最大摩擦力为

B2L2v0
2R
+mgsinθ;(2)导体棒EF上升的最大高度为
m
v20
-4Q
2mg

阅读理解

The US government has started a website, Admongo, to help children think critically about the advertising aimed at them. It claims to provide visitors with an “education” through games and other amusement.

A cartoon man dressed in old time pilot clothing greets visitors to Admongo. “Call me Haiz”, he says upon arrival in a rocket ship that opens up with a crazy world inside it. Spacey dance music plays in the background as Haiz tells visitors that they need to learn about advertising.

Its inventors say eight to twelve years old is the age kids develop their critical thinking abilities. Kids that age are also a big market for advertisers.

The idea behind Admongo is to teach children three things: To identify the advertiser. To know what the advertiser is really saying. And to know what the advertisement is trying to get the child to do.

Children learn these things through a video game. They create their own game character. They can choose different skin colors, hair styles, eye and mouth shapes. Then they begin a trip through ad-land, where there are ads on buses and billboards. The players have to find all the marketing in the neighborhood before they can move on to the next level.

The Admongo game takes players inside a home, to the advertising studio and everywhere else ads can be found. It is a complete exploration of the world of marketing.

One such area is food marketing. The Federal Trade Commission (FTC) says it is a big business. The FTC estimates that food, drink and fast-food restaurants spent more than one and a half billion dollars on advertising to young people in 2010.

The FTC says children are important for three reasons. They buy products. They influence parents and caregivers to buy. And they are the future adult buyers of the products.

A recent study says most advertising aimed at children is for foods of the lowest nutritional value. First Lady Michelle Obama has said she would like to see advertisers marketing healthy foods for children.

小题1:What is the best title of the text?

A.The guide of Admongo

B.An education website for children

C.A popular online video game

D.A website aimed at children小题2:Why did the government start the website?

A.To attract the biggest market of buyers.

B.To sell the products of its company.

C.To help children know about advertising.

D.To advertise the video game for children.小题3:The underlined word “identify” can be replaced by _________.

A.recognize

B.inform

C.persuade

D.forbid小题4:What can players do in the website game?

A.Choose hair styles for their character.

B.Travel to a supermarket.

C.Eat in a fast-food restaurant.

D.Play video games during the trip.小题5:Children are important for advertising because they are _________.

A.important for the society

B.the most potential buyers

C.easily influenced by ads

D.easily affected by poor products

单项选择题