试题与答案

如图所示是一个设计“过山车”的试验装置的原理示意图,光滑斜面AB与竖直面内的圆形

题型:计算题

题目:

如图所示是一个设计“过山车”的试验装置的原理示意图,光滑斜面AB与竖直面内的圆形轨道在B点平滑连接,圆形轨道半径为R。一个质量为m的小车(可视为质点)在A点由静止释放沿斜面滑下,当它第一次经过B点进入圆形轨道时对轨道的压力为其重力的7倍,小车恰能完成圆周运动并第二次经过最低点沿水平轨道向右运动。已知重力加速度为g

(1)求A点距水平面的高度h;  

(2)假设小车在竖直圆轨道左、右半圆轨道部分克服摩擦阻力做的功相等,求小车第二次经过竖直圆轨道最低点时的速度大小。

答案:

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答案:A

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Does a Dog Help a Teen with Low Self-esteem?

A dog is known as man’s best friend for good reason. Loyal and affectionate ,a dog companion can enrich its owner’s life in many ways, 1_____ Many teenagers suffer from periods of low self-esteem (自尊)as they try to navigate the wavy waters of adolescence and deal with all of its associated highs and lows.

A teenager who lacks self-esteem may feel isolated from his peers and unable to tell his parents about personal issues. The company of a dog can be a huge comfort and ease feelings of loneliness and awkwardness. A dog offers genuine unconditional love. 2_____

All dogs need exercise,so taking his clog for a walk is the perfect opportunity for a teenager to get outdoors and benefit from fresh air and physical activity. Exercise allows people to take control of their bodies,which is often the first step toward regaining control of other aspects of their life,says mental health specialist Dr. Alan Cohen. 3    Exercise releases chemicals called endorphins (内排肽)into the brain? which create positive responses in the body ,including improved self­esteem.

Dogs can help teenagers be more sociable and make new friends, suggests HELPGUIDE. 4_____ A dog is a great ice-breaker ,avoiding awkward social situations and putting the teenager at ease with people he meets. The more confident the teen becomes when out and about with his clog ,the more his self-esteem will improve.

5_____ If a teenager is suffering from depression or developing dangerous habits such as alcohol and drug abuse ,more extreme action may be required. In these cases ,professional help should be sought and all treatment plans considered. Parents who suspect that their teen’s low self­esteem is a sign of a far more serious problem should ask a suitably qualified health professional for advice.

A.In some cases, having a dog will not help with a teenager’s low self-esteem.

B.It simply accepts its owner for who he is and doesn’t judge, question or criticize.

C.Pets offer an unconditional love that can be very helpful to people with depression.

D.You don’t have to worry about hurting your pet’s feelings or getting advice you don’t want.E. In particular, someone with low self-esteem may find a pet dog to be extremely beneficial.

F. A teen may meet his peers while walking his dog in the park or taking part in a dog training class.

G. Many forms of exercise besides walking can be enjoyed with a dog, such as cycling, jogging and throwing a ball.

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