题目

有一玩具汽车绝缘上表面固定一个带负电物块，它们的总质量m=0.5kg，物块带电量q= —5．0×10-5c。现把玩具汽车放置在如图所示的水平直轨道A点，BC由光滑管道弯曲而成的半圆轨道，玩具汽车在光滑管道中能自由运动，整个轨道所处空间存在竖直向下的匀强电场，其电场强度大小E=6.0×l04N／c。玩具汽车在水平直轨道运动时阻力恒为Ff=0.5N，通电后玩具汽车以恒定功率P=l0w行驶，通电1.0s自动断电，断电后玩具汽车能以一定的速度从B点进入半圆轨道。已知AB Spring is here. Did you plant trees on Tree-Planting Day? Everyone knows that climate change is a serious problem these days. People have a lot of work to do in order to prevent it. One of the best ways is to plant more trees.According to a study by NASA, the Earth’s green leaf area has increased by 5 percent, or over 5 million square kilometers since 2000. China is responsible for 25 percent of the increase. That’s about one square of the size of the whole Amazon rainforest.Over the past twenty years, China has made great efforts to green the land in order to fight soil erosion (水土流失) and air pollution. It is one of the most successful countries in greening the desert.For example since 1962, China had been planting trees in Saihanba in Hebei province. It was once a barren(贫瘠) area. After overcoming many difficulties, foresters have turned the area into China’s largest man-made forest. Another example is the Kubuqi Desert, which is near the Yellow River in Inner Mongolia. It was once the source(来源) of sandstorms in Beijing. But now, about one third of the Kubuqi Desert is covered with trees, and sandstorms are less common. Mu Us Desert was also known for its sandstorms many years ago. But now, one-quarter of the desert has been turned into forest.In a word, planting trees is really important to us. Let’s take part in planting trees to make our earth greener and more beautiful.1.The reason why China makes great efforts to green the land is to _____A. prevent climate change B. plant more treesC. reduce water pollution D. stop the soil from being washed away2.Which of the following is TRUE according to the passage?A. China succeeds in greening the desertB. Planting tree is the best way to prevent the climate changeC. The green leaf area of China has increased by 0.25 million square kilometers since 2000.D. Mu Us Desert cause the sandstorms in Beijing3.It is _______ for foresters to turn Saihanba into the forestA. dangerous B. exciting C. difficult D. impossible