题干

空降兵某部官兵使用新装备进行超低空跳伞,若跳伞空降兵在离地面224m高处,由静止开始在竖直方向做自由落体运动.一段时间后立即打开降落伞,以12.5m/s2的加速度匀减速下降.为了空降兵的安全,要求空降兵落地速度最大不得超过5m/s.则空降兵打开降落伞时离地的高度至少为多少米?(g取10m/s2,不计空气阻力)

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答案(点此获取答案解析)

解:设空降兵做自由落体运动的高度为h时速度为v,此时打开伞开始匀减速运动,落地时速度刚好为5m/s,这种情况空降兵在空中运动时间最短,则有:

v2=2gh,vt2﹣v2=2a(H﹣h)

解得:h=125m,v=50m/s

为使空降兵安全着地,他展开伞时的高度至少为:H﹣h=224m﹣125m=99m

答:则空降兵打开降落伞时离地的高度至少为99m

同类题2

根据短文内容,从短文后的选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。

    Homework is a major part of going to school: It's your teachers' way of evaluating(评价) how much you understand of what's going on in class. ____ Luckily, you can do a few things to make homework less work.

    Be sure you understand the homework.

    Write your homework down in your notebook or day planner if you need to. ____ It's much easier to take a minute to ask the teacher during or after class than to struggle to remember later that night!

    Use any extra time in school.

    Many schools have study halls that are specifically designed to allow students to study. It's tempting (诱惑) to hang out with friends during study periods or unstructured (松散的) time. ____  

    Take a break.

    ____ So take some breaks while doing your homework. Sitting for too long without stretching or relaxing will make you less productive than if you stop every so often. Taking a 15-minute break every hour is a good idea for most people.

    ____

    If you don't finish your homework during school, think about how much you have left and what else is going on that day, and then budget your time. Most high-school students have between 1 and 3 hours of homework a night. If it's a heavy homework day and it seems like you've got an assignment in every subject but gym and lunch, you'll need to devote more time to homework. It's a good idea to come up with some kind of homework schedule, especially if you are involved in sports or activities or have an after-school job.

A. Pace yourself.

B. Settle down to do homework.

C. And it helps digest important concepts (概念).

D. Most people's attention spans (注意力持续时间) aren't very long.

E. Don't be afraid to ask questions about what's expected.

F. Once your homework is done, you can check over it if you have extra time.

G. But the more work you can get done in school, the less you'll have to do that night.