题目:水平固定的光滑U型金属框架宽为l,足够长,其上放一质量为m的金属棒ab,左端连接有一阻值为R的电阻(金属框架、金属棒及导线的电阻均可忽略不计),整个装置处在向下的匀强磁场中,磁感应强度大小为B.现给棒一个初速v0,使棒始终垂直框架并沿框架运动,如图1所示.(1)金属棒从开始 Title: The horizontally fixed smooth U-shaped metal frame has a width of l and is long enough. A metal rod ab of mass m is placed on it, and a resistance of R is connected to the left end (resistances of the metal frame, metal rod, and wire can all be Neglecting), the entire device is in a downward uniform magnetic field, and the magnetic induction intensity is B. Now give the rod a initial velocity v0, so that the rod always moves vertically and moves along the frame, as shown in Figure 1. (1) Metal rods From the beginning