题目

阅读理解 Nearly20 U.S. states have started carrying out former president Barack Obama's Clean Power Plan, which places limitson carbon dioxide emissions from power plants in an effort to reduce theimpacts of climate change. The plan has been in legal limbo (边缘) for the past year. Yet scientists havenow calculated another outcome of the policy: harm to crop yields (产量) ifthe plan is cancelled. Along with carbon pollution, coal-fired power plantsspew (喷出) pollutants that form what we know assmog. The contribution of smog to increased rates of asthma (哮喘) andpremature deaths w6．如图所示，水平向左的匀强电场E=4V/m，垂直纸面向里的匀强磁场B=2T，质量m=1g的带正电的小物块A．从M点沿绝缘粗糙的竖直壁无初速下滑，滑行0.8m到N点时离开竖直壁作曲线运动，在P点时小物块A瞬时受力平衡，此时速度与水平方向成450，若P与N的高度差为0.8m，g=10m/s2，求：（1）A沿壁下滑过程中摩擦力所做的功；（2）P与N的水平距离．