题干

演绎式探究
风洞实验室中可以产生方向水平、速度大小可调节的风,用来研究处在流动气体中物体的受力情况.将一个套有木球的细直杆放入风洞实验室,已知球受到的重力为G.球在杆上运动时,球与杆之间的滑动摩擦力跟它们之间的压力成正比,比例系数是k.如图甲所示,将杆沿平行风速的方向固定,调节风速的大小,使球恰能沿杆以速度v匀速运动.再将杆沿竖直方向固定,如图所示,将球从杆的上端由静止释放,保持球下滑过程中所受风力与杆处于水平方向时受到的风力大小相同.(提示:运动物体所具有的动能公式为
m
v
2
2
,其中m代表物体的质量,v代表物体的运动速度.)则:
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同类题3

    阅读下面四篇短文,从每题所给的A、B、C、D四个选项中选出最佳选项。

B

    Recently,there was a report about a special app called "notOK".A 13-year-old girl Hannah Lucas and her brother Charlie put their heads together to create this app last year.

    Hannah has been suffering from an illness that causes her to pass out since she was little.This is very dangerous if there is no one around.She was also being bullied(欺凌)by some students at school.So she searched for an app that would allow her to press a button so that a friend or a family member would know immediately that she needed help.After realizing there was no such thing,Hannah and her brother decided to do something about it.After some research,Hannah made some early drawings of what the app might look like.Then,after taking a programming class,Charlie began to think about the best way for the app to work.

    They wanted something that would send a text message to up to five contacts that reads:"Hey,I'm not OK.Please call me,text me,or come and find me",along with a link to the user's current GPS location.

    They started looking for someone to help them make the app.And when Hannah shared her dream with people in a summer business class which she attended two years ago,they were really excited about the app.They thought of many different marketing ideas.By mid—December last year, "notOK" app was finished as a birthday present for Hannah from Charlie."It was exactly what we wanted,"Charlie said.

    In today's age,most people will agree that apps like these are great.Whether you're suffering from illness,or anything emergent(紧急的),writing a text message or making a phone call to get the help you need can be difficult.

    Hannah is happy to know that her app will help more people."It's like I went through my dark times so I could help people through theirs,"she said.