A rod of mass m and resistance R slides smoothly over two parallel conducting wires kept sloping at an angle theta with respect to the horizontal as shown in figure. The circuit is closed through a perfect conductor at the top. There is a constant magnetic field vecB along the vertical direction. If the rod is initially at rest, find the velocity of the rod as a function of time.

A rod of mass m and resistance R slides smoothly over two parallel con

1.	A rod of mass m and resistance R slides smoothly over two parallel conducting wires kept sloping at an angle theta with respect to the horizontal as shown in figure. The circuit is closed through a perfect conductor at the top. There is a constant magnetic field vecB along the vertical direction. If the rod is initially at rest, find the velocity of the rod as a function of time.
Answer» Solution :When the pebble is dropped , then initially its VELOCITY increases . After sometime it becomes constant. The viscous FORCE acting on pebble through viscous oil `F=6pietarv` (where r= radius of pebble , v = velocity of pebble and `eta` = coefficient of viscosity) As the viscous force is variable force ,hence acceleration is also variable so `vtot` graph will not be a straight LINE. Hence , option (C ) is correct.