题干

关于功率公式和P=Fv的说法正确的是(    )

A:据,知道W和t就可求出任意时刻的功率

B:由P=Fv只能求某一时刻的瞬时功率

C:从P=Fv知汽车的功率与它的速度成正比

D:从P=Fv知当功率一定时,牵引力与速度成反比

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答案(点此获取答案解析)

D

同类题1

阅读下文,回答下列问题。

守候一株水仙

朱钟洋

    那年春天,我把母亲从乡下接进城来同住。一进门,母亲便看到了窗台上的花,满心欢喜。窗台上,葱兰、芦荟、吊兰、茉莉……在暖暖的春风里,长得郁郁葱葱,娇艳欲滴的叶子沐浴在阳光里。

    忽然,母亲看着窗台一角的花说:“这盆花都蔫了,真可惜。”母亲久居乡下,不识得那盆花。那是年前妻在县城一家花店,特意买的水仙,就因它能赶在过年前开花,能给我们的新年增添一抹淡香,让我们感受到生活的富贵和灿烂。如今,已经是阳春三月了,水仙花期早过,它的枝叶散了,耷拉着,渐渐腐朽、干枯。水仙已经走过了生命里最美的时光,赖以生存的根须已被掏空了养分。

    我安慰母亲说:“这是水仙,花开花谢之后就变成这样了。因为我们这里的气候土壤不适合水仙生长,它以后再也不会开花了。您看到的那些开花的水仙都是从外地买过来的。”说完,我用手轻轻一提,水仙便从沙土里提出来了,一粒粒细沙稀稀落落地粘在水仙的根须上。

    我刚要把水仙扔掉,母亲慌忙制止我:“好歹也是生命,怎能随手扔掉呢?”母亲抢过水仙,像宝贝似的把它种在一个闲置的大花盆里,还用黑泥把它的根须盖好,压实。

    看着母亲执著的样子,我没有再说什么,只能任由母亲去折腾。

    母亲把水仙摆在窗台最靠近阳光的地方,日复一日地浇水、施肥、松土、除草。可尽管母亲费尽心思,它还是渐渐蔫下去,直到落尽最后一片叶子。

    我以为,只要水仙落尽了叶子,母亲便会不再搭理它,可母亲仍旧日复一日地呵护它。在母亲心里,只要水仙一息尚存,就还有希望。

    夏天过去了,秋天也接近了尾声,母亲告诉我说:“你知道吗,水仙又冒出绿芽了。”

    我点点头,看着母亲高兴的样子,笑了。我知道,即便水仙长出了绿叶,也是一株野草,不会再开花了。

    果然,一切如我所料,一年又一年过去了,水仙只管冬日里长出茂密的叶子,然后春日里悄然落尽。尽管水仙从未放弃过生命,但它丝毫也没有开花的迹象。

    又是一年春草绿,当我忙完了一天的工作回到家的时候,母亲惊喜地告诉我:“水仙开花了!”我走近窗台,凝望着那株水仙,我惊呆了——就在一丛水仙叶子的中间,单单一朵水仙花恣意地绽放着,如从天而降的仙女,超凡脱俗。

    在我们这里种植的水仙不是不能开花吗?这株水仙是怎么开花的?我惊讶得张大了嘴巴,连呼吸都屏住了。我的眼睛一眨也不敢眨地看着水仙花,生怕转眼之间这位落入凡间的仙女就要离我而去。

    一株被带离故土的水仙,花开花谢之后,在快成一株干草的时候,被一位疼爱它的人拾起,小心浇灌,倍加怜惜。想不到,身处他乡的水仙,在主人日复一日年复一年的守候中竟然也能绽放出最美的花朵,以独有的姿态展现自己的价值。

    守候一株水仙,它是不会让人失望的;守候一株水仙,就是守候一个顽强的生命,就是守候一颗执著开花的心。我们的身边是不是也有一个疼爱自己的人在守候?我们的生命是不是也能像一株水仙?少一些抱怨,多一些坦然;少一些嫉恨,多一些珍爱;少一些冷漠,多一些感动;珍惜生命里所有的好,让生命极致绽放,一次,又一次,再一次……

(选自《思维与智慧》2015年第9期)

同类题4

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    Everybody hates it, but everybody does it. A recent report said that 40%of Americans hate tipping. In America alone, tipping is a $ 16 billion-a-year industry. Consumers acting politely ought not to pay more than they have to for a given service. Tips should not exist. So why do they? The common opinion in the past was that tips both rewarded the efforts of good service and reduced uncomfortable feelings of inequality. And also, tipping makes for closer relations. It went without saying that the better the service, the bigger the tip.

    But according to a new research from Cornell University, tips no longer serve any useful function. The paper analyzes numbers they got from 2,547 groups dining at 20 different restaurants. The connection between larger tips and better service was very weak. Only a tiny part of the size of the tip had anything to do with the quality of service.

    Tipping is better explained, by culture than by the money people spend. In America, the custom came into being a long time ago. It is regarded as part of the accepted cost of a service. In New York restaurants, failing to tip at least 15% could well mean dissatisfaction from the customers. Hairdressers can expect to get l5%-20%, and the man who delivers your fast food $ 2. In Europe, tipping is less common. In many restaurants the amount of tip is decided by a standard service charge. In many Asian countries, tipping has never really caught on at all. Only a few have really taken to tipping. According to Michael Lynn, the Cornell papers' author, countries in which people are more social or outgoing tend to tip more. Tipping may reduce anxiety about being served by strangers. And Mr. Lynn says, “In America, where people are expressive and eager to mix up with others, tipping is about social approval. If you tip badly, people think less of you. Tipping well is a chance to show off.”