题干

滑雪运动深受人民群众喜爱,某滑雪运动员(可视为质点)由坡道进入竖直面内的圆弧形滑道AB,从滑道的A点滑行到最低点B的过程中,由于摩擦力的存在,运动员的速率不变,则运动员沿AB下滑过程中(   )

A:所受合外力始终为零

B:所受摩擦力大小不变

C:合外力做功一定为零

D:机械能始终保持不变

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C

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Reader's Travel Photography Competition

    This month's images include some original and brilliant shots. The overall 2018 winner will receive a $200 holiday ticket and go on a 16-night wildlife holiday.

Doug Scott

Guess what the young monk is doing! He is playing with his smart phone in secret.

PAUL GOLDSTEIN JUDGE: Smart phones have taken over the world. Even without the photographer's caption there is composition, humor and fun here. To the outsider, he could be having a short sleep, or lost in meditation. My eyes stop on this for a while and the admission of smart phones cannot be changed in any corner of the world.

Eloise Campbell

I was able to follow this young eagle hunter and his eagle in the mountains of Mongolia and watched how it was trained. Though the bond between them was new, to me it seemed unbreakable—they were coexisting peacefully with each other.

Adam Cunningham White

This shot grasps the moment when we had to change direction with this group of around 300 migrating reindeer in northern Sweden. They can become confused very quickly, making this moment risky. They could run in different directions immediately, making it difficult to get them back into a group.

Nick Dale

This is a close-up of the left eye of a zebra.

PAUL GOLDSTEIN JUDGE: Perfect, I have seen these before but rarely done with this competence. The depth of field is exactly right. The bold cropping and blue to the left make people interested. Zebras are easy to photograph but not often this well.