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 40.0 g and electrical resistance 0.200 ? rests on parallel horizontal rails that have negligible electric resistance. The rails are a distance L= 7.00 cm apart. (Figure 1)The rails are also connected to a voltage source providing a voltage of V = 5.00 V . The rod is placed in a vertical magnetic field. The rod begins to slide when the field reaches the value B = 0.112T . Assume that the rod has a slightly flattened bottom so that it slides instead of rolling. Use 9.80 m/s2 for the magnitude of the acceleration due to gravity.

Find ?s, the coefficient of static friction between the rod and the rails. Give your answer numerically.