如图所示,一根固定的绝缘竖直长杆位于范围足够大且相互正交的电场和磁场中,电场强度大小为E= ,磁感应强度大小为B。一质量为m、电荷量为q的带正电小圆环套在杆上,环与杆间的动摩擦因数为μ o 现使圆环以初速度v o 向下运动,经时间t o ,圆环回到出发点。若圆环回到出发点之前已经开始做匀速直线运动,不计空气阻力,重力加速度为g。则下列说法中正确的是( )
【简答题】Hobbs was an orphan(孤儿). He worked in a factory and every day he got a little money. Hard work changed him thin and weak. He wanted to borrow a lot of money to learn to paint pictures, but he did not ...