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问题 选择题

如右图中的水平桌面足够长，不计托盘质量和滑轮与绳的摩擦．物体A重10N，当物体B重为3N时，物体A向右做匀速直线运动，要使物体A向左做匀速直线运动，则应对A物体施加一个水平向左的力，其大小为（   ）

A．3N

B．6N

C．10N

D．13N

答案

答案：B

当物体B重为3N时，物体A向右做匀速直线运动，所以A受到的摩擦力是3N，要使A向左做匀速直线运动，对A进行受力分析，A受到向右的摩擦力，大小等于3N，受到线对A向右的拉力，所以必须对A施加一个向左的6N的拉力。

阅读理解
阅读理解。
A. defensive B. common C. fact D. fear E. consciousness
F. values G. rush H. questioned  I. unlikely J. unusually

     Ten years after the attacks on the World Trade Center and the Pentagon on September 11, 2001, America
has come out of the smoke a very different country. Nearly 3,000 people died on September 11.
     Suddenly America had to wake up to the    1   that not everyone loves the USA. Members of al-Qaeda, the
militant group that carried out the attacks, were angry about US support for the Israelis against Palestinians in
the Middle East, about US troops in Saudi Arabia, and about sanctions (制裁) against Iraq.
     "September 11 and the years that followed were a shock to our national    2  ," said Nicholas Burns,
American ambassador to NATO at the time. The struck not only concrete and steel, but also Americans' faith
in their power. The   3  that America had lost control of events was widespread.
     Historian Douglas Brinkley said 9/11 put America into a   4  "crouch". It started a mad   5    for protection.
The government agreed to measures that pulled apart privacy.
     In the name of fighting terrorism, the US launched wars in Iraq and Afghanistan. More than 6,000
Americans have died. Tens of thousands of ordinary people in those countries have also lost their lives. But
casualties are only part of the story. The world was shocked by photographs of Americans torturing Iraqis in
Abu Ghraib prison in Baghdad. David Goldstein, a reporter with The Kansas City Star,    6    American
behavior:"Has torturing so-called terrorists saved American lives or made the    7   we trumpet around the
world weaker?…Is that who we've become?" However, a brief moment of national unity did occur straight
after 9/11."Americans were coming together in an    8  powerful way… in the ashes. We live in a bittersweet
memory of that collective tragedy and collective possibility," said filmmaker Ken Burns. September 11 was
that kind of   9   moment. When it was over, the Earth still turned in its usual way and the stars in the sky
shone as before. But the universe had shifted somehow.

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单项选择题

乙类火燃烧特点是（）.

A.火能深入内部易燃

B.燃烧炽烈，火势

C.表面燃烧且速度快温度高易爆炸
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