A proton travels through uniform magnetic and electric fields. The magnetic field is in the negative x direction and has a magnitude of 2.70 mT. At one instant the velocity of the proton is in the positive y direction and has a magnitude of 1950 m/s. At that instant, what is the magnitude of the net force acting on the proton if the electric field is (a) in the positive z direction and has a magnitude of 4.55 V/m, (b) in the negative z direction and has a magnitude of 4.55 V/m, and (c) in the positive x direction and has a magnitude of 4.55 V/m?