How Does the Millikan Oil Drop Experiment Illustrate the Balance of Forces?

[General_Sax]

Homework Statement

An oil drop with a mass of 7.20*10^-16kg moves upward at a constant speed of 2.50 m/s between two horizontal, parallel plates. If the electric field strength between these plates is 2.20*10^4 V/m, what is the magnitude of the charge on the oil drop?

Homework Equations

F = ma
Fg = mg
Fe = qE
E = V/d

The Attempt at a Solution

F = ma
F = m(0)
Fnet = 0

Fnet = Fe - Fg
Fg = Fe
q = mg/E

q = 3.2*10^-19C

My problem is conceptual. I don't really understand how the particle can have a constant velocity and a net Force of zero upon it.

How did the particle accelerate from stationary to it's current velocity?

 

[LowlyPion]

The history of the particle isn't all that important. The mention of constant velocity is to reinforce the idea that it is not accelerating, and hence no net force.