问题 计算题

如右图所示,光滑斜轨和光滑圆轨相连,固定在同一个竖直面内。圆轨的半径为R,一个小球(质量为m,大小可忽略不计)从离水平面高h处由静止开始自由下滑,由斜轨进入圆轨。

(1)若小球到达圆轨最高点时对圆轨的压力大小恰好等于自身重力大小,那么小球过圆轨最低点时对圆轨的压力是多大?

(2)为了使小球在圆轨内运动的过程中始终不脱离圆轨,

h应在什么范围内取值?

答案

(1)设球在圆轨最高点时速度大小为v2, 应有    

2mg=mv2 2/R  ……………………①(2分)

设球在圆轨最低点时速度大小为v1从圆轨最低点到最高点过程机械能守恒,

mv12/2=" 2" mgR+mv2 2/2     ………② (2分)

球过最低点时,有

F—mg= mv12/R              …………③ (2分)

联立以上几式解得:   F="7mg   " (2分)

(2)2.5R

(3)

单项选择题

For some people, the light of human attention has an unbearable brilliance. Like ivy along the dim edge of a garden, they prefer the social shadows, shunning parties, publicity and fame of any sort. Then there are the flowers of the human arboretum. For them, being in the view of others seems necessary for life itself. From Hollywood to fabricated prime-time reality, this spotlight-dependent species is thriving.
But what about the individuals who crave attention for more desperate reasons Those who resort to unusual ways to get it Lately, it seems, a dark bloom of these characters has emerged. For motives known only to themselves, they have won notoriety by drawing on an almost sacred well of social status: victim hood.
In early April, US national news outlets tracked the disappearance of Audrey Seiler, a sophomore at the University of Wisconsin in Madison. Police and hundreds of concerned citizens searched for four days before Seiler was discovered. Seiler said she was kidnapped. Within hours, however, her story fell apart. Police announced that her abduction had been a hoax. Why would a popular student make herself disappear Her motive remains a mystery, but perhaps it had something to do with the search parties and the news bulletins that surrounded her.
Sympathy is a powerful sentiment that can connect complete strangers. But if it’s used to manipulate, the backlash can be much more intense.
In February, a Waterbury, Connecticut, man was arrested as a result of exploiting sympathy. Edward Valentin told reporters that he had received word that his wife, serving in Iraq, had been killed in an explosion. Police said Valentin admitted the fabrication, reasoning that if people felt sorry for him maybe the military would send his wife home. Evidence, however, points elsewhere.
In its extreme form, such a craving shows up in mental disorders, where sufferers may seek attention by causing themselves harm. But even when it comes with no diagnosis, a deep craving to be noticed can have a wide impact.
For these individuals, victim hood represents a "pure state of guilt-free entitlement," said psychologist Richard Levak, of Del Mar, California. "They go from being utterly deprived to being utterly indulged. In today’s world ... people have become more depressed and disconnected from each other. So you get people who crave affection and attention and approval. They don’t know how to ask for it and they don’t know how to get it. That leaves them vulnerable," Levak said.

The word "notoriety" underlined in Paragraph 2 is closest in meaning to ______.

A.infamy

B.reclame

C.reputation

D.achievement

单项选择题 A1型题