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 10.0g and electrical resistance 0.500? rests on parallel horizontal rails that have negligible electric resistance. The rails are a distance L = 9.00cm apart. (Figure 1) The rails are also connected to a voltage source providing a voltage ofV = 5.00V .
The rod is placed in a vertical magnetic field. The rod begins to slide when the field reaches the value B = 5.44

Explanation / Answer

Current I in the rod = V/R = 5/0.5*10^-2 = 10 A
Length through which B crosses = 0.09 m
Magnetic field B = 5.44e-2T

Force acting on the rod = ILB = 10*0.09*5.44e-2 = 0.048 N

Frictional force = ?mg
Net force = 0.048 - ?mg = 0 when motion just begins.

? = 0.048 / (0.01*9.8) = 0.48

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