FBD stands for "Free Body Diagram", so I am assuming you are describing some type of ride that starts by storing up gravitational potential energy into the vehicle you will be riding in. This is most commonly done by elevating the vehicle some distance above where it will finally wind up. When the vehicle is released, gravity starts the vehicles acceleration downward, at a rate of 9.8 meters/second/second. So you would need to calculate how fast the vehicle is going as it nears the ground. That would tell you how much force would need to be generated by your magnetic braking system. If the magnetic braking system failed, what would be your safeguard to absorb all the kinetic energy of the vehicle and it's occupants? Don't forget friction; friction would be a great way to absorb some of the energy. One of the best free body drops I experienced was a ride at Six Flags over Georgia, where the vehicle was dropped from a height of six stories! Our hearts were in our throats by the time the ride was over!