A copper rod of mass m slides under gravity on two smooth parallel rails, with separation $l$ and set at an angle of $\theta$ with the horizontal. At the bottom, rails are joined by a resistance $R$. There is a uniform magnetic field $B$ normal to the plane of the rails, as shown in the figure. The terminal speed of the copper rod is: [Figure] (1) $\frac { \mathrm { mgR } \cos \theta } { \mathrm { B } ^ { 2 } l ^ { 2 } }$ (2) $\frac { \mathrm { mgR } \sin \theta } { \mathrm { B } ^ { 2 } l ^ { 2 } }$ (3) $\frac { \mathrm { mgR } \tan \theta } { \mathrm { B } ^ { 2 } l ^ { 2 } }$ (4) $\frac { \mathrm { mgR } \cot \theta } { \mathrm { B } ^ { 2 } l ^ { 2 } }$
A copper rod of mass m slides under gravity on two smooth parallel rails, with separation $l$ and set at an angle of $\theta$ with the horizontal. At the bottom, rails are joined by a resistance $R$. There is a uniform magnetic field $B$ normal to the plane of the rails, as shown in the figure. The terminal speed of the copper rod is:\\
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(1) $\frac { \mathrm { mgR } \cos \theta } { \mathrm { B } ^ { 2 } l ^ { 2 } }$\\
(2) $\frac { \mathrm { mgR } \sin \theta } { \mathrm { B } ^ { 2 } l ^ { 2 } }$\\
(3) $\frac { \mathrm { mgR } \tan \theta } { \mathrm { B } ^ { 2 } l ^ { 2 } }$\\
(4) $\frac { \mathrm { mgR } \cot \theta } { \mathrm { B } ^ { 2 } l ^ { 2 } }$