题干

手机已经成为了人们日常必备品之一,低头看手机的“低头族”随处可见(图甲).有研究发现,若玩手机时姿势不当,可能让颈椎承受多达27公斤的重压.为求证这一问题,某同学查阅相关资料,画出了“低头族”头部前倾某一角度时的模型(图乙),颈椎看成轻杆,颈部肌肉看成轻绳,头部看成质点,所受重力已标出.请在答卷的图丙中用平行四边形定则作出重力与颈部肌肉对头部拉力的合力,并根据所作的图测量颈椎承受的压力是头部重力的倍.(保留两位有效数字)

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解答:重力与颈部肌肉对头部拉力的合力沿颈椎方向,根据平行四边形定则作出合力如图所示,该合力等于颈椎承受的压力,根据测量知,合力的大小大约等于重力的2.6倍,则颈椎承受的压力是头部重力的2.6倍(2.6~2.9 均对).

同类题2

阅读下面短文,掌握其大意,从每题所给的A、B、C、D四个选项中,选出最佳选项。

    If life were a book and you were the author, how would you like the story to go? That is the question that 1 my life forever.

    One day I went home from the training of snowboarding with what I thought was the flu, and less than 24 hours later, I was in a 2 on life support with less than two percent 3 of living. It wasn't until days later that the doctors diagnosed me with a 4 blood infection.

    Over the 5 of 2.5 months, I lost the hearing in my left ear and both my legs below the knee. When my parents 6 me out of the hospital, I 7 that I had been put together like a patchwork(拼缝物) doll and I had to live with 8 legs. I was absolutely physically and emotionally broken, 9 streaming down.

    But I knew in order to move forward, I had to let go of the Old Amy and 10 the New Amy. It was at this moment that I asked myself that significant 11. And that is when it 12 me that I didn't have to be five-foot-five anymore,13 I could be as tall as I wanted. And 14 of all, I can make my feet the size of all the shoes. So there were 15 here.

    Four months later, I was back upon a 16. And this February, I won two Board World Cup gold medals, 17 me the highest ranked snowboarder in the world.

    So, instead of looking at our 18 and our limitations as something 19 or bad, we can begin looking at them as a wonderful 20 that can be used to help us go further than we ever know we could go.