Question: What is the radius of the smallest possible circular orbit that a 2MeV proton can have in a 2 T magnetic field? Answer (what I've got so far, at least): I assume that r = mv/qB is the formula I use for this problem, due to being given mass, B, q and all that, but I can't figure out how to apply it. I assume that v sould be replaced with an equation, but I cannot find that equation. Plus, it doesn't appear that there can be a "smallest" radius because as long as v is above zero, there will be a radius, but v can get infinitely small. Any suggestions?