搜题请搜索小程序“爱下问搜题”
问题 问答题

最初斜面和木箱均静止，后来木箱自光滑斜面滑下，如图16-10所示，木箱和斜面的质量分别为m="10" kg和M="50" kg，斜面长L="2" m,不计斜面与地面的摩擦，斜面倾角为30°.求木箱滑至斜面底部时斜面移动的距离.

图16-10

答案

取木箱和斜面为一个系统研究，在水平方向上无外力作用，此系统的动量在水平方向上是守恒的.

设斜面后退的距离为x,则木箱移动的水平距离为Lcos30°-x,根据水平方向动量守恒知

M·x=m(Lcos30°-x),

整理得

.

阅读理解

阅读理解。

     Movies are great things. They have a long time. They tell us many things.

There are funny movies, scary movies and also exciting movies. Different

people like different kinds of movies. Comedies are welcomed by the people of all ages. Usually old people also like Beijing Operas and documentaries.

Young people, especially(尤其) boys like action movies and thrillers. They

also like western movies. In western movies they learn something about the

life of the cowboys（牛仔） in western countries, like the USA. Go to

movies in the cinema or stay at home to watch them on TV. You can learn a

lot.

1. People, old and young, like _________.

A. documentaries  

B. comedies  

C. Beijing Opera  

D. romances

2. ______ like thrillers best.

A.Girls  

B. Boys  

C. Old people  

D. All the children

3. “Western” means(意思是)___.

A.  西部的  

B. 科幻的  

C. 动画的  

D. 东部的 

4. Western movies are about the life of the _________ boys.

A. Chinese  

B. school    

C. American    

D. English

5. People ______ if they want to see movies.

A. watch them on TV    

B. go to the cinemas  

C. go to the concert      

D. A and B
查看答案
填空题

基因工程又叫基因拼接技术。

(1)在该技术中，用人工合成方法获得目的基因的途径之一是：以目的基因转录的______为模板，_____成互补的单链DNA，然后在酶的作用下合成_______。

(2)基因工程中常用的受体细胞有细菌、真菌、_______。若将真核基因在原核细胞中表达，对该目的基因的基本要求是________。

(3)假设以大肠杆菌质粒作为运载体，并以同一种限制酶切割运载体与目的基因，将切割后的运载体与目的基因片段混合，并加入DNA连接酶。连接产物至少有____种环状DNA分子，它们分别是_________。
查看答案