如图所示,电阻不计的粗糙金属轨道MN与M′N′平行放置,轨道间距L=2m,轨道平面与水平方向成37°角。同种材料、粗细均匀的正方形金属框abcd边长也为L,a、d两点通过铰链与轨道连接。在外力F
0(F
0未知)的作用下,使金属框abcd以ad为转轴顺时针匀速转动,转动角速度ω=1rad/s。t=0时刻,ab边、cd边分别与轨道重合,此时水平放置的导体杆ef在导轨上静止释放。已知导体杆ef与轨道之间的动摩擦因数μ=
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmfrac%3E%3Cmn%3E3%3C%2Fmn%3E%3Cmn%3E4%3C%2Fmn%3E%3C%2Fmfrac%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
(最大静摩擦力等于滑动摩擦力),且杆ef与ad间距足够远。金属框abcd质量
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmsub%3E%3Cmi%3Em%3C%2Fmi%3E%3Cmn%3E1%3C%2Fmn%3E%3C%2Fmsub%3E%3Cmo%3E%3D%3C%2Fmo%3E%3Cmfrac%3E%3Cmrow%3E%3Cmn%3E6%3C%2Fmn%3E%3Cmi%3E%CF%80%3C%2Fmi%3E%3C%2Fmrow%3E%3Cmrow%3E%3Cmn%3E49%3C%2Fmn%3E%3C%2Fmrow%3E%3C%2Fmfrac%3E%3Cmi%3Ek%3C%2Fmi%3E%3Cmi%3Eg%3C%2Fmi%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
,各边电阻均为1Ω;杆ef质量m
2=
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmfrac%3E%3Cmn%3E1%3C%2Fmn%3E%3Cmrow%3E%3Cmn%3E14%3C%2Fmn%3E%3C%2Fmrow%3E%3C%2Fmfrac%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
kg,电阻为1Ω;空间存在平行于轨道且斜向上的匀强磁场,磁感应强度B=0.5T。(sin37°=0.6,cos37°=0.8)