题干

在军事演习中,某空降兵从飞机上跳下,先做自由落体运动,在t1时刻,速度达到最大值v1时打开降落伞,做减速运动,在t2时刻以较小速度v2着地.他的速度图像如图3所示.下列关于该空降兵在0~t1或t1~t2时间内的平均速度 v¯ 结论正确的是(   )

A:0~t1v¯v12

B:t1~t2v¯ = v1+v22

C:t1~t2v¯v1+v22

D:t1~t2v¯v1+v22

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D

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    “Our aim is to take our art to the world and make people understand what it is to move,” said David Belle, the founder of parkour(跑酷).

    Do you love running? It is a good exercise, yet many people find it boring. But what if making your morning jog a creative one? Like jumping from walls and over gaps, and ground rolls? Just like the James Bond in the movie Casino Royale? Bond jumps down from a roof to a windowsill and then runs several blocks over obstacles on the way. It is just because of Bond's wonderful performances that the sport has become popular worldwide.

    Yes, that's parkour, an extreme street sport aimed at moving from one point to another as quickly as possible, getting over all the obstacles in the path using only the abilities of the human body. Parkour is considered an extreme sport. As its participants(参与者) dash around a city, they may jump over fences, run up walls and even move from roof top to roof top.

    Parkour can be just as exciting and charming as it sounds, but its participants see parkour much more than that.

    Overcoming all the obstacles on the course and in life is part of the philosophy(理念) behind parkour. This is the same as life. You must determine your destination, go straight, jump over all the barriers as if in parkour and never fall back from them in your life to reach the destination successfully. A parkour lover said, “I love parkour because its philosophy has become my life, my way to do everything.”

    Another philosophy we've learnt from parkour is freedom. It can be done by anyone, at any time, anywhere in the world. It is a kind of expression of trust in yourself. It is only a state of mind. It is when you trust yourself that you earn energy.