题干

某汽车在紧急刹车时加速度大小是6m/s2.如果必须在2s内停下来,汽车的行驶速度最高不能超过多少?若车以72km/h行驶,在紧急刹车时,4s内的位移是多少?

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答案(点此获取答案解析)

解:⑴采用逆向思维,则汽车行驶的最大速度为:

v=at=6×2m/s=12m/s.

⑵72km/h=20m/s,停止运动的时间为:

t′= va=206

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    When Abraham Lincoln was young, he worked in a store. One day a woman came into the store and bought some things. They added up to two dollars and six and a quarter cents.

    The bill was paid, and the woman was satisfied. But the young storekeeper, not feeling quite sure about his calculations(计算), added up the things again. To his surprise, he found that it should have been but two dollars.

    “I've made her pay six and a quarter cents more,” said young Abe, upset.

    It was an unimportant thing, and many salespersons would forget it, but Abe was too careful for that.

    The money must be paid back, he decided.

    At night, he closed the store and walked to the home of his customer. He explained the matter, paid over the six and a quarter cents, and returned satisfied.

    Here is another story of young Lincoln's strict honesty. A woman entered the store and asked for half a pound of tea. The young man weighed it out. This was the last sale of the day.

    The next morning, when beginning his duties, Abe discovered a four-ounce weight on the scales(磅砰). It flashed upon him at once that he had used this in the sale last night, and so, of course, given his customer short weight. Many people would not have been much worried by this discovery. But Abe weighed out four ounces of the tea, shut up the store, and carried it to the customer. I think that the name, so often given in late times to President Lincoln, of “Honest Old Abe", was well deserved(应得的).

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        半个月后,弟弟果然进了那个学校,家里人一派喜庆,都夸父亲厉害,父亲得意地一扬眉毛:“啥事我都能摆平,没有难倒我的事情。”
        没多久,弟弟就知道自己是高价生,是钱买的而不是条子批的了,母亲追问是怎么回事,父亲掩饰不住了,只得坦白:“我把多年收集的邮票卖了,交了‘建校费’……”
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        其实,事情是这样的:父亲以前的一个老同事,在电信做副总,在一次婚宴上,两个现今职位悬殊的曾经的同事喝着喝着就喝高了,副总还让我父亲喝,我父亲肚子里在揣摩着母亲的事情,立即说道:“如果你能给我爱人安排个事情做,这半斤酒我一口气喝完!”喝高了的副总立即答应,结果,父亲就喝进了医院。
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