问题 选择题

如图所示,倾角为60°的斜面固定在水平面上,轻杆B端用铰链固定在竖直墙上,A端顶住质量为m、半径为R的匀质球并使之在图示位置静止,此时A与球心O的高度差为

R
2
,不计一切摩擦,轻杆可绕铰链自由转动,重力加速度为g,则有(  )

A.轻杆与水平面的夹角为60°

B.轻杆对球的弹力大小为2mg

C.斜面对球的弹力大小为mg

D.球所受的合力大小为mg,方向竖直向上

答案

A、设轻杆与水平方向夹角为θ,由几何知识sinθ=

h
R
=
1
2
,θ=30°,故A错误;

B、对球受力分析如图:

由几何知识得:轻杆对球的弹力大小N2=mg,故B错误;

C、如上图,由几何知识可得斜面对球的弹力大小为mg,C正确;

D、球处于静止,平衡状态,所受的合力大小为0,D错误;

故选:C.

阅读理解

阅读理解。

    Listening test is one of the important parts of the English exam. Here are some tips for you.

    Before you start to listen, you need to relax. Don't be nervous. And try to read the questions. These

questions usually help you understand the dialogue or the passage.

    Then listen carefully to the first sentence. It usually tells you the main idea of the passage. When

you're listening, try to do some thinking and take some notes (笔记), such as:

    ·What happened?

    ·When, where, and how?

    ·What does the speaker want to tell us?

In this way, you may understand the passage better.

     Please remember not to think about one or two words for a long time. When you hear some words

you don't know, don't spend too much time on them. Very often, you'll find out what they mean later

when you go on with the listening.

1. This passage mainly talks about ______. [ ]

A. tips on listening

B. tips on speaking

C. tips on reading

D. tips on writing

2. It's better for you to keep ______ when you start to have a listening test. [ ]

A. excited

B. relaxed

C. nervous

D. worried

3. We should listen to the first sentence carefully, because it usually tells us            . [ ]

A. the answers to the questions

B. where to write the answers

C. the main idea of the passage

D. how much time left (剩余) for the listening test.

4. When you're listening, it's important for you to ______. [ ]

A. remember every word

B. take some notes

C. understand all the words

D. think about one words for a long time

5. The             usually help(s) us understand the passage better. [ ]

A. questions

B. first sentence

C. notes

D. A, B and C

论述题