Determining the direction of magnetic field

[darksyesider]

http://ocw.mit.edu/courses/physics/8-02sc-physics-ii-electricity-and-magnetism-fall-2010/faradays-law-1/faradays-law/MIT8_02SC_challenge_sol21.pdf


In problem 8 b), they say the magnetic force must cancel with gravity. How do you determine which way the magnetic force is?
I don't understand this conceptually, although I get how to solve the problem from there.

 

[Dale]

The terminal speed is by definition the speed where a=0. Then Newtons 2nd law gives you the net force and the net force tells you that the magnetic force cancels with gravity.