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 50.0 g and electrical resistance 0.500 rests on parallel horizontal rails that have negligible electric resistance. The rails are a distance L = 8.00 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 = 0.306 T . 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.

s =

Explanation / Answer

Current I in the rod = V/R = 5/0.5 = 10 A
Length through which B crosses = 0.08 m
Magnetic field B = 0.306 T

Force acting on the rod = ILB = 10*0.08*0.306 = 0.2448 N

Frictional force = mg
Net force = 0.2448 - mg = 0 when motion just begins.

= 0.2448/ (0.05*9.8) = 0.49959 = 0.5

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