A rail gun uses electromagnetic forces to accelerate a projectile to very high v

Question

A rail gun uses electromagnetic forces to accelerate a projectile to very high velocities. The basic mechanism of acceleration is relatively simple and can be illustrated in the following example. A metal rod of mass 40.0g and electrical resistance 0.500? rests on parallel horizontal rails that have negligible electric resistance. The rails are a distance L = 10.0cm apart. (Figure 1) The rails are also connected to a voltage source providing a voltage of V = 5.00V .
The rod is placed in a vertical magnetic field. The rod begins to slide when the field reaches the value B = 0.196T . Assume that the rod has a slightly flattened bottom so that it slides instead of rolling. Use 9.80m/s2for the magnitude of the acceleration due to gravity.

Find ?s, the coefficient of static friction between the rod and the rails.

I got .4 the first time I tried.  

Explanation / Answer

F = iLB = 10 * 0.1 * 0.196 = 0.196 N

SInce it is moving with constant velocity

hence

2 umg (because of 2 horizontal rails) = 0.196

2 * u * 0.04 * 9.8 = 0.196

u = 0.196/(9.8 * 0.04 * 2) = 0.25

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