Conducting rail ciruit, against gravity

[Frostfire]

Homework Statement

So my textbook has a lot of example of conducting rails moving horizontally with unknown variables of F and B, but how would you approach a problem that has such a device pointed vertically with the moving bar droping? let's say R and B are known and we need to find I(current) sufficient to counteract gravity so it doesn't accelerate past a velocity required to generate that current.

Homework Equations

Fb=qv x B
R = delta V/I
Fg = mg (call the mass of the rod m )
I = p/delta V

The Attempt at a Solution

I played with it, but not sure if I am right.The part I am stuck on is what relation of the magnetic field do I set equal to Fg

is it Fg= qV x B

 

[Frostfire]

So I found a formula for magnetic levitation that allows me to relate magnetic field strength and gravity

F= IlB=mg

if I am right this allows me to determine the current needed to counteract gravity.Does this sound right? Anybody no where that formula is derived from?

 

[Frostfire]

Can somebody give me some advice here? Its kind of hard to ask questions that aren't in the book, if your unsure of what your doing