问题 问答题

如图所示,倾角θ=30º、宽度L=1m的足够长的“U”形平行光滑金属导轨固定在磁感应强度B =1T,范围足够大的匀强磁场中,磁场方向垂直于斜面向下。用平行于轨道的牵引力拉一根质量m =0.2㎏、电阻R =1Ω的垂直放在导轨上的金属棒a b,使之由静止开始沿轨道向上运动。牵引力做功的功率恒为6W,当金属棒移动2.8m时,获得稳定速度,在此过程中金属棒产生的热量为5.8J,不计导轨电阻及一切摩擦,取g=10m/s2。求:

(1)金属棒达到稳定时速度是多大?

(2)金属棒从静止达到稳定速度时所需的时间多长?

答案

(1)v =" 2m/s            "

(2)t =1.5s     

(1)金属棒沿斜面上升达稳定速度时,设所受的安培力为F,由平衡条件得: 

F=mgsinθ+F                       

F=BIL=B L                

                          

联立以上三式解得v =" 2m/s              "

(2)由能量转化与守恒定律可得

Pt = mgssinθ++Q          

代入数据解得:t =1.5s        

单项选择题
阅读理解

阅读下面短文和问题,根据短文内容,在相应题号后的横线上写下相关信息(不多于五个单词),

完成对该问题的回答。答语要结构正确,书写工整,字迹清楚。

     Human activities have great effect on the environment. Everyone can make small life changes toward

a low-carbon (低碳) life. Such changes can not only make a difference for the environment, but also have

great health benefits for you.

     ● Burn less gas. Try walking or bicycling to work a few times a week and get a healthy exercise on the

way. Encourage children to walk to school. Cutting down on driving and using your own two legs helps

cut global warming pollution and helps deal with weight problems over time.

     ● Eat more thoughtfully. Eat less meat and more grains and vegetables, which helps reduce the need to

change forests or grasslands to pasturelands (牧场). Replace one or more of your weekly meals with a

meatless meal.

     ● Drink tap water rather than buy bottled water. Bottled water produces up to 1.5 million tons of plastic

waste per year. Bottled water is not necessarily cleaner than tap water, and tap water is cheaper.

     ● Reduce the use of paper. Think twice before you print. If you must print, try printing on both sides.

     ● Reduce the amount of water you use in your home. Turn off the water while brushing your teeth and

turn off the shower after soaping up. Wash your fruits and vegetables in a basin of water instead of running

water from the tap.

     ● Use energy-saving lamps to save money, use less energy and create less pollution. These lamps are easily

available and have various styles and brands.

回答下面问题,在相应题号后的横线上写下相关信息(不多于五个单词)。

1. How can we eat more thoughtfully?

    __________________________________________________________________________

2. If we have to print something, what should we do?

    __________________________________________________________________________

3. What is the passage mainly about? 

    __________________________________________________________________________