如图所示，一长为2L的轻杆中央有一光滑的小孔O，两端各固定质量_爱下问搜题

搜题请搜索小程序“爱下问搜题”

问题选择题

如图所示，一长为2L的轻杆中央有一光滑的小孔O，两端各固定质量为2m和m的A、B两个小球，光滑的铁钉穿过小孔垂直钉在竖直的墙壁上，将轻杆从水平位置由静止释放，转到竖直位置，在转动的过程中，忽略一切阻力．下列说法正确的是（　　）

A．杆转到竖直位置时，A、B两球的速度大小相等，为

gL

3

B．杆转到竖直位置时，杆对B球的作用力向上，大小为

mg

3

C．杆转到竖直位置时，B球的机械能减少了

4mgL

3

D．由于忽略一切摩擦阻力，A球机械能一定守恒

答案

A、将轻杆从水平位置由静止释放，转到竖直位置时，A、B两球的速度大小相等，设为v，根据系统的机械能守恒，得

2mgL-mgL=

1

2

•3mv2，解得，v=

2

3

gL

．故A错误．

B、杆转到竖直位置时，对B球：mg-N=m

v2

L

，解得，N=

mg

3

．故B正确．

C、杆转到竖直位置时，B球的重力势能增加为mgL，动能增加量为

1

2

mv ²=

1

3

mgL，则其机械能增加了

4

3

mgL．故C错误．

D、尽管忽略了一切摩擦阻力，但杆对A球做功，由上分析可知，A球的机械能减小．故D错误．

故选B

阅读理解

阅读短文,根据短文内容补全表格中所缺的信息.(每空一词)

    Now you've finished three years of English study. There's one way to demonstrate(展示)what you've
learned-take a test!
    Do you have any special methods(方法) you use to help you do better on a test? How well you do is often a matter of how well prepared you are. Here are some ways to make sure you're at your best on test
days.
    Go to bed early. The night before a test is not the time to stay up watching TV. You want to be rested
and wide-awake for test day.
    Have a good breakfast. Test day isn't a morning for donuts(面包圈).Nor is it a time to starve yourself. Eat plenty of low-sugar, high-energy food to keep your brain and body working their best.
    Be prepared, Make sure you have two pencils and anything else you were asked to bring.
    Watch the numbers. Make sure that the number of the items(项目)on your answer sheet matches the
number of the question.
    Look over all the answer choices(选择). Don't assume(假定)that choice A is correct until you've checked the others. You may find a better one. And it is wrong to eliminate(排除)any choice that you can see at once. You'll have a better chance of finding the right answer.
Check the clock. Make sure you allow yourself enough time to answer every question and go back and
check your answers .Don't spend too much time on any difficult question .Skip(跳读)it and come back to
it later.
    Keep up with your school work all year long That's how you get the skills you need for taking tests.

How to make sure that you can do 1 on test days
2__	Things you do
Going to bed early	Not 3____ up or being asleep for test day
Having a good breakfast	Eating __4___low-sugar, high-energy food
Being prepared	Remembering to 5_ two pencils and the 6 things you were asked to bring
Watching the numbers	The number of your answer should 7 the number of the question
Looking over all the choices	Don't assume that the choice A is correct 8_ you've checked the others
Checking the clock	9_ your time carefully and not spending too much on any difficult question
You will get the skills you need 10_ you keep up with your schoolwork all year long

单项选择题

下列各项中，在办理支付结算时应当承担第一付款责任的是( )

A．办理托收承付的银行

B．办理委托收款的银行

C．办理汇兑的银行

D．开具信用证的银行