如图所示，两平行金属导轨间的距离L=0.40m，金属导轨所在的平面_爱下问搜题

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问题计算题

如图所示，两平行金属导轨间的距离L=0.40m，金属导轨所在的平面与水平面夹角θ=37°，在导轨所在平面内，分布着磁感应强度B=0.50T、方向垂直于导轨所在平面的匀强磁场。金属导轨的一端接有电动势E=4.5V、内阻r=0.50Ω的直流电源。现把一个质量m=0.040kg的导体棒ab放在金属导轨上，导体棒恰好静止。导体棒与金属导轨垂直、且接触良好，导体棒与金属导轨接触的两点间的电阻R₀=2.5Ω，金属导轨电阻不计，g取10m/s²。已知sin37°=0.60，cos37°=0.80，求：

（1）通过导体棒的电流；

（2）导体棒受到的安培力大小；

（3）导体棒受到的摩擦力。

答案

解：（1）导体棒、金属导轨和直流电源构成闭合电路，根据闭合电路欧姆定律有：

I==1.5A

（2）导体棒受到的安培力：F_安=BIL=0.30N

（3）导体棒所受重力沿斜面向下的分力F₁= mgsin37°=0.24N

由于F₁小于安培力，故导体棒受沿斜面向下的摩擦力f

根据共点力平衡条件mgsin37°+f=F_安

解得：f=0.06N

完形填空

It’s known that the 62-year-old Jim Bevier has a great passion. This former FedEx pilot wasn’t ____ on his horse farm, teaching his grandkids to ____. In his spare time, Bevier helps distribute ____ for blindness around the globe.
Bevier now is a volunteer pilot for Orbis, a nonprofit organization which has fought blindness for over 25 years. One of ____ weapons is the Flying Eye Hospital, an old-fashioned DC-10 aircraft. Bevier’s mission is to land this giant sight-saving bird on runways that are most ____ not designed for it. “It’s not ____ flying for FedEx,” says Bevier. “We have to ____ the runway is long enough, figure out the fuel load, and find the ____ place to park in case of accidents.”
Once he gets the ____ to its destination, the aircraft becomes a place for local ____ to get training, so they can treat certain conditions and ____ blindness for people. The plane stays on location for weeks. ____ Bevier is free to return home, he likes to stay and ____ those who arrive for medical checks. “I’m glad to see them get on the airplane and come out the next day, and they can ____.” Bevier says. Many walk on dirt roads for miles; many are blind children.
____, the volunteer doctors can’t handle all the patients. “A Vietnamese boy was ever ____ away.” Bevier says. A pilot’s job involves hours and hours of boredom, so you learn to keep your ____ under control. But when the poor boy didn’t get picked, I ____ down.” he admits.
____ as the job may be, Bevier isn’t looking into retirement anytime soon. “I don’t play golf, and I tell my kids not to buy me tennis shoes,” he says. “But I ____ of my grandchildren back home, and all I want is for those kids to be able to see their own grandparents for the first time.”

C．differently

B．Challenging

C．Disappointing

单项选择题

从绍兴寄往重庆的EMS，重821克，邮件资费为（）。

A.24元

B.29元

C.26元

D.20元