A particle (mass = 4.0 g, charge = 80 mC) moves in a region of space where the electric field is uniform and is given by Ex = −2.5 N/C, Ey = Ez = 0. If the velocity of the particle at t = 0 is given by vx = 80 m/s, vy = vz = 0, what is the speed of the particle at t = 2.0 s?
A)40 m/s
B)20 m/s
C)60 m/s
D)80 m/s
E)180 m/s

Question

Quizplus · Accepted Answer

First, find the acceleration of the particle using F = ma and the electric force equation:
F = qE = (80 x 10^-3 C)(-2.5 N/C) = -0.2 N
a = F/m = -0.2 N/ (4 x 10^-3 kg) = -50 m/s^2 (note the negative sign indicates a force in the negative x direction)
Then, use the kinematic equation v = vo + at to find the velocity at t = 2 s:
vx = 80 m/s (constant velocity in y and z directions)
vy = 0 m/s
vz = 0 m/s
v = sqrt(vx^2 + vy^2 + vz^2) = sqrt((80 m/s)^2) = 80 m/s (magnitude of velocity remains constant in the absence of a force in the y and z directions because the motion is only in the x direction) 
Finally, the speed is the magnitude of velocity so the answer is 80 m/s.

A Particle (Mass = 4