题目

根据短文内容，判断对错。Do you remember the story of Cinderella? Cinderella comes from Germany(德国). It is a story from Grimm 's Fairy Tales(《格林童话》). The Grimm brothers started to collect(收集)fairy tales in 1806. Their first book came out in 1812.Why are German fairy tales so interesting? Maybe it is because they come from the Black Forest. The Black Forest is very famous. It is a big and beautiful forest in Germany. It is famous for its trees and lovely views(景观). There are valleys and waterfalls（山谷和瀑布）there. It's a good place to start a story. 8．如图所示，在平面直角坐标系xOy的第一象限内有一个磁感应强度大小B=0.2T的矩形区域的匀强磁场（图中未画出），其方向垂直于xOy平面向里；第二象限有一匀强电场，电场强度的大小E=2×103 V/m．在y轴上的P点处有一个质量m=4×10-20kg、电荷量q=2×10-14C的带正电粒子垂直于y轴以V0=3×104 m/s的速度沿x轴的正方向射入第一象限，粒子飞出磁场区域后又恰好回到y轴上的P点，再以垂直于电场的方向进入第二象限，最后垂直打在x轴上的A点．已知粒子在电场中经历的时间t=$\sqrt{3}$×10-5s，A点坐标为（-$\frac{3\sqrt{3}}{40}$m，0），粒子的重力不计，求：（结果可保留根号）（1）粒子在磁场中做圆周运动的半径r和从P点进入第二象限时速度与y轴负方向的夹角θ．（2）粒子在xOy平面的第一象限内运动的时间t0．（3）粒子在矩形磁场区域内做圆周运动的圆心位置坐标．