题干

“10米折返跑”的成绩反应了人体的灵敏素质.测定时,在平直跑道上,受试者以站立式起跑姿势站在起点终点线前,当听到“跑”的口令后,全力跑向正前方10米处的折返线,测试员同时开始计时,受试者到达折返线处时,用手触摸折返线的物体(如木箱),再转身跑向起点终点线,当胸部到达终点线的竖直面时,测试员停表,所用时间即为“10米折返跑”的成绩.设受试者起跑的加速度为4m/s2,运动过程中的最大速度为4m/s,快到达折返线处时需减速到零,减速的加速度为8m/s2,返回时达到最大速度后不需减速,保持最大速度冲线.求该受试“10米折返跑”的成绩为多少秒?

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答案(点此获取答案解析)

解:对受试者,由起点终点线向折返线运动的过程中
加速阶段据运动学公式得:t1=
v
m
a
1
=1s;
s1

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阅读理解

    Recently, I experienced a wonderful lesson in how little things still mean a lot. My brother, mother and I live in a very rural district of Hawaii. Our farm is at least a dozen miles from the most basic of services. Therefore, I take weekly trips to COSTCO to get supplies. About a month ago, I finished loading up the car and was about to leave, when a piece of paper caught my eye. I picked it up and read it carefully.

    It was a receipt(收据) from the State Motor Vehicle Division, recording the owner's payment of her Vehicle's Registration fees. At first I thought that I could find the owner. So I waited there for about an hour. Although the receipt had been borne (由……携带) on the wind, where in the busy, crowded parking lot would I find the owner? I looked over the receipt for contact or any personal data, perhaps a license tag(牌照)or telephone number. I checked the date, the fees paid, noted the name of the owner and pocketed the paper. I concluded that the best and easiest step to take was to put the receipt in an envelope and send it to the owner first the next morning.

    By the end of the week, I received a beautiful thank-you letter from a very grateful and happy woman containing a handwritten message and a gift card. In the letter, the woman explained how the wind snatched(夺去) her receipt from a pocket in her car's passenger door. She had searched everywhere for quite some time before giving up.

    It felt great to know I'd helped someone avoid a loss by doing something that at first glance(一瞥) seemed little or even unimportant.