The force required to start an object sliding across a uniform horizontal surface is larger than the force required to keep the object sliding at a constant velocity once it starts.
The magnitudes of the required forces are different in these situations because the force of kinetic friction is less than the force of static friction. <em>(d)</em>
B.Electromagnetic
explanation:
F = Magnetic Force
B = Magnetic Field
V = Velocity
*The vectors from the photo you get doing the left-hand rule.
The magnetic force is always perpendicular to the magnetic field.
And as told in the statement, the electron is moving perpendicular to a magnetic field, that is, the Velocity forms an 90 degree angle / Right angle with the magnetic field.
The formula to find the Magnetic Force is:

Where "q" is the Charge and the sin theta is the angle formed by the Velocity and Magnetic Field, in this case it's 90°. Sin 90° = 1.

Newton (N) = C x m/s x T = (C x m x T)/s
V = l x w x h
v = 3 x 6 x 4
v = 72