A sort of "projectile launcher" is shown in Fig. 20-65. A large current moves in a closed loop composed of fixed rails, a power supply, and a very light, almost frictionless bar touching the rails. A magnetic field is perpendicular to the plane of the circuit. The bar has a L = 20 cm, a mass of 1.8 g, and is placed in a field of 1.7 T. I wrote my knowns and unknowns but I'm lost on what formulas to use.