问题 选择题

如图所示,质量分别为m1和m2的两个小球A、B,带有等量异种电荷,通过绝缘轻弹簧连接,置于绝缘光滑的水平面上.当突然加一水平向右的匀强电场后,两小球A、B将由静止开始运动,在以后的运动过程中,对两个小球和弹簧组成的系统(设整个过程中不考虑电荷间库仑力的作用且弹簧不超过弹性限度),以下说法正确的是(   )

A.系统机械能不断增加

B.系统动能不断增加

C.当弹簧长度达到最大值时,系统机械能最小

D.当小球所受电场力与弹簧的弹力相等时,系统动能最大

答案

答案:D

题目分析:据题意,在施加电场后对整个系统受力分析,系统在水平方向受到电场力F1和F2,由于两球带电量相等且电性相反,则在匀强电场中,系统在竖直方向的合力也为0,所以系统所受合力为0;弹簧弹力是系统的内力,两球在电场力作用下向相反方向运动,弹簧被拉伸,弹力增加,当弹力与电场力相等时,两球速度最大,继续运动则弹力大于电场力,两球开始减速运动直至速度减为0,然后反向运动;整个系统在这个运动过程中,系统机械能守恒,A、C选项错误;动能先增加后减少,B选项错误;D选项正确。

阅读理解

National parks are large of areas of public land . They give a safe home for local plants and animal . They help keep the air and water clean . They also give us the best trips . Today ,there are nearly 7,000 national parks around  the world . Forbes has listed 12 of the most beautiful ones in the world that will surprise  you with their  amazing landscape ,geographic wonders and colorful plants and animals .

The Grand Canyon National Park of the US is one of the Seven Natural Wonders of the World . It is best known for its size and depth . It is 446 kilometers long ,up to 29 kilometers wide ,and 1. 6 kilometers deep . The immensity of the canyon makes people think big . Every year ,about 5million people visit here . Taking a tour in the South Rim offers vistors the park’s full views,while the North Rim shows beautiful wild flowers .

If you love animals ,you’ll like the Serrengeti Natinoal Park in Tanzania . The “Big Five ”live here . They are the lion . African elephant ,African leopard ,black rhinoce and African buffalo . The name  “Big Five ”came from the five animals that were the hardest to catch . Now they are what people most want to see in the wild . If you visit the park ,you should never miss the migration of over 1. 5million wildebeests and 250,000 zebras every year. This is the most famous site of the park .

小题1:Which is NOT right about national parks according to Paragraph 1?

A.Anational park is public land that covers a large area.

B.Wild plants and animals are protected in national parks.

C.Forbes has listed twelve of the world’s largest national parks.

D.National parks are attractive because of their natural beauty.小题2:The underlined word“immensity”refered to_______.

A.great size

B.natural wonder

C.colorful view

D.unusual beauty小题3:______kinds of animals are mentioned in the passage.

A.Five

B.Six

C.Seven

D.Eight小题4:The best title for the passage may be “_______”.

A.Taking a tour in the Grand Canyon

B.Travelling to the Serengeti National Park

C.Seven National Wonders of the World

D.Stepping into the wonders of the wildlife小题5:If the writer continues the article ,he would most likely write about _______.

A.ways to keep the air and water clean

B.more national parks in the world

C.his wonderful trip to the Grand Canyone

D.famous historic wonders in Africa

单项选择题 A1/A2型题