Why can't the magnetic force on an electron be determined from B, v, and q?

[dcgirl16]

an electron moves within a uniform magnetic field of .2T at a speed of 5x10^5 m/s. What is the magnetic force on the electron?

I used Fm=qvBSin and got 1.6x10^-14N but the answer is that it can't be determined with this info why not?

 

[nrqed]

an electron moves within a uniform magnetic field of .2T at a speed of 5x10^5 m/s. What is the magnetic force on the electron?

I used Fm=qvBSin and got 1.6x10^-14N but the answer is that it can't be determined with this info why not?

Because you don't know the angle between the velocity of the electron and the magnetic field! You are right that the magnitude of the magnetic field is $F_m = |q| v B sin \theta$ but without the angle $\theta$ you can't find the answer. (how did {\em you} get an answer? I bet that you used an angle equal to 90 degrees!)

Patrick