【简答题】A heartless thief is believed to have crashed a fund-raiser and made off with a bag of cash meant to help a New York City firefighter pay for life-changing surgery for his 9-year-old son. But little A...
【单选题】如图所示,空间某区域中有一匀强磁场,磁感应强度方向水平,且垂直纸面向里,磁场上边界b和下边界d水平.在竖直面内有一矩形金属线圈,线圈上下边的距离很短,下边水平.线圈从水平面a开始下落.已知磁场上下边界之间的距离大于水平面a、b之间的距离.若线圈下边刚通过水平面b、c(位于磁场中)和d时,线圈所受到的磁场力的大小分别为F b ,F c 和F d ,则