What Determines the Smallest Orbit Radius for a Proton in a Magnetic Field?

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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?

 

[James R]

The 2 MeV is the kinetic energy of the proton. So, you can work out the velocity v of the proton from the kinetic energy. You also know the charge q on a proton, and you're given B. You'll need the proton mass m, too.

In calculating v, you'll need to convert the energy in MeV into an energy in Joules.