A proton travels a short distance in an electric field, then it enters a crossed magnetic field of 1 T and radius 0.2 m. Find the velocity of the proton.

The proton follows a circular path in the presence of magnetic field B. Hence the centripetal force is given by the magnetic force.
⇒ mv²/r = qvB
⇒ v = qBr/m = (1.6×10⁻¹⁹)(1)(0.2)/(1.67×10⁻²⁷) = 0.2×10⁸ m/s