题干

起重机将物体由静止举高h时,物体的速度为v,下列各种说法中正确的是(不计空气阻力)(  )

A:拉力对物体所做的功等于物体动能的增量

B:拉力对物体所做的功等于物体动能和势能的增量

C:拉力对物体所做的功等于物体势能的增量

D:物体克服重力所做的功大于物体势能的增量

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答案(点此获取答案解析)

B

同类题2

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    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.