如图(a)所示,平行金属导轨MN、PQ光滑且足够长,固定在同一水平面上,两导轨间

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问题:

如图(a)所示,平行金属导轨MN、PQ光滑且足够长,固定在同一水平面上,两导轨间距L=0.25m,电阻R=0.5Ω,导轨上停放一质量m=0.1kg、电阻r=0.1Ω的金属杆,导轨电阻可忽略不计,整个装置处于磁感强度B=0.4T的匀强磁场中,磁场方向竖直向下,现用一外力F沿水平方向拉杆,使其由静止开始运动,理想电压表的示数U随时间t变化的关系如图(b)所示.试分析与求:

(1)分析证明金属杆做匀加速直线运动;

(2)求金属杆运动的加速度;

(3)写出外力F随时间变化的表达式;

(4)求第2.5s末外力F的瞬时功率.

考点:牛顿第二定律电功电功率闭合电路欧姆定律导体切割磁感线时的感应电动势
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某公司2008年购进一处房产,2009年5月1日用于投资联营(收取固定收入,不承担联营风险),投资期3年,当年取得固定收入160万元。该房产原值3000万元,当地政府规定的减除幅度为30%,该公司2009年应缴纳的房产税为( )。

A.21.2万元
B.27.6万元
C.29.7万元
D.44.4万元

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Shopping for clothes is not the same experience for a man as it is for a woman. A man goes shopping because he needs something. He knows what he wants, and his purpose is to find it and buy it, the price is a less important consideration. All men simply walk into a shop and ask the assistant for what they want. If the shop has it, the salesman immediately produces it, and the business of trying it on is done at once. All being well, the bargain (交易) can be and often is completed in less than five minutes, with hardly any chat and to everyone’s satisfaction.

For a man, slight problems may begin when the shop does not have what he wants. Then the salesman tries to sell the customer (顾客) something else he offers the nearest he can to the thing required. He usually says, "I know this jacket is not the style (式样) you want, sir, but would you like to try it on for size It happens to be the color you wanted." Few men have patience with this treatment, and the usual reply is: "This is the right colour and may be the right size, but I should be wasting my time and yours by trying it on."

Now how does a woman go about buying clothes She does so in the opposite way. She has never fully made up her mind what she wants, and she is only "having a look round". She is always open to persuasion, indeed she considers of great importance what the saleswoman tells her, even what her friends tell her. She will try on any number of things. Highest in her mind is the thought of finding something that everyone thinks suits her. Most women have an excellent sense (判断力) of value when they buy clothes. They are always trying to find an unexpected bargain. Faced with a roomful of dresses, a woman may easily spend an hour going from one counter (柜台) to another before selecting the dresses she wants to try on. It takes a great deal of time but gives great joy. Most dress shops supply chairs for the waiting husbands.

When a man is buying clothes, he ().

A. puts price before quality

B. chooses things that others think suitable for him

C. does not mind much the price he has to pay for the fight things

D. buys good quality things, so long as they do not cost too much

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