Zero Magnetic Force: Proton in Uniform Magnetic Field

[dpogre]

Homework Statement

A proton with an initial velocity of magnitude v in the +x direction enters a uniform magnetic field. For which direction or directions of the magnetic field, if any, will there be zero magnetic force on the proton?
+ x
- x
+ y
- y
+ z
- z

Homework Equations

There were no equations i could think of that will help we with this problem, other than the open palm thing that my professor talked about in class today.

The Attempt at a Solution

I don't really understand the question. on the graph (+) x is in the third quadrant and that's the velocity. I tried -x by itself, i tried -x, +y, and +z. And +y, +z. All three were incorrect. I don't think i understand what it means to have zero magnetic force on a proton.

 

[rl.bhat]

Force on the charged particle is given by F = qvBsin(theta) where theta is the angle between the velocity and the magnetic field.
Now you can decide when the force is zero.