问题 选择题

两个完全相同的小球A和B,质量均为m,用长度相同的两根细线悬挂在水平天花板上的同一点O,再用长度相同的细线连接A、B两小球,如图所示。然后用一水平向右的力F作用于小球A上,使三线均处于直线状态,此时OB线恰好位于竖直方向,且两小球都刚好静止,两小球均可视为质点,则力F的大小(   )

A.0

B.mg

C.mg

D.mg

答案

答案:B

题目分析:对A进行受力分析如图,此时绳子AB刚好绷直,拉力为零,所以根据几何关系有T和水平方向夹角为30º,则有,则

故选B

点评:此题关键点是绳子AB没有拉力, AB若有拉力就不能保证OB竖直了。

单项选择题
单项选择题

In the end, a degree of sanity prevailed. The militant Hindus who had vowed to breach a police cordon and start the work of building a temple to the god Ram at the disputed site of Ayodhya decided to respect a Supreme Court decision barring them from the area. So charged have Hindu-Muslim relations in India become in recent weeks, as the declared deadline of March 15th neared, that a clash at Ram’s supposed birthplace might well have provoked bloodshed on an appalling scale across the nation. It has, unfortunately, happened often enough before.
But the threat has not vanished. The court’s decision is only an interim one, and the main Hindu groups have not given up on their quest to build their temple. Extreme religious violence, which seemed in recent years to have faded after the Ayodhya-related explosion of 1992--1993, is again a feature of the political landscape. Though faults lie on both sides (it was a Muslim attack on Hindus in a train in Gujarat that started the recent slaughter), the great bulk of victims were, as always, Muslims. Once again, educated Hindus are.to be heard inveighing against the "appeasing" of Muslims through such concessions as separate constitutional status for Kashmir or the right to practice Islamic civil law. Once again, the police are being accused of doing little or nothing to help Muslim victims of rampaging Hindu mobs. Once again, India’s 130m Muslims feel unequal and unsafe in their own country. Far too many Hindus would refuse to accept that it is “their own country" at all.
The wonder of it, perhaps, is that things are not worse. While the world applauds Pakistan for at last locking up the leaders of its extreme religious groups, in India the zealots still support, sustain and to a degree constitute the government. The BJP, which leads the ruling coalition, was founded as a political front for the Hindu movement. It is simply one, and by no means the dominant, member of what is called the Sangh Pariwar, the "family of organizations". Other members of the family are much less savoury. There is the VHP, the World Hindu Organization, which led the movement to build the Ram temple. There is the Bajrang Dal, the brutalist "youth wing" of the VHP. There is substantial evidence that members of the VHP and the Bajrang Dal helped to organize the slaughter of hundreds of Muslims in Gujarat after 58 Hindus were killed on a train as they returned from Ayodhya.

Towards the issue of Hindu-Muslim relations, the writer’s attitude can be said to be
[A] objective.
[B] biased.
[C] appalled.
[D] supportive.