I am not able to understand the following situations. In stokes' experiment a tiny lead shot falls freely under gravity in a highly viscous column of liquid. When the viscous force becomes equal to the net weight of the lead shot it is said that the shot moves down with a constant velocity known as terminal velocity. The explanation given is that the net force on the shot becomes zero at this point as the upward acting viscous force on the shot cancels the net down ward force due to the weight of the shot. That is the acceleration of the lead shot becomes zero which means the increase in velocity of the shot becomes zero. So the lead shot, instead of hanging at rest at that point itself, actually moves down with a constant terminal velocity. Now think of this situation. In Millikan's oil drop method,an oil drop is initially made to fall under gravity in between two plates through air.Here also the oil drop attains the constant terminal velocity under gravity when the net force on the oil drop is zero. But when you apply a suitable upward acting electric field on the charged oil drop, you can alter the field in such a way that the drop either moves up or stand still in air. When the drop is made to stand still without moving either up or down, again we take that the net force acting on the drop is zero. Here, we take the net weight of the drop is equal to the force due to the electric field,ie. (mg = Eq) while solving problems. My doubt is this: In the previous case also the net force on the lead shot is zero but the shot did not stop and stay still but continued to move downwards with a constant velocity where as in the second case how is that the charged oil drop could be made to stand still by making the net force acting on it zero? Then which statement is true, ie. when the net force on a freely falling body through a viscous medium becomes zero, it continues to move down but with a constant velocity (first case), or when the net force on a freely falling body through a viscous medium becomes zero, it just stops and stands still there itself (second case)?