V depends on the particular machine and on the speed of rotation. 10 -13 sec. Its S value was known before its function or molecular weight, and so has become its name. The molecules of larger S value go farther because of eithe r larger molecular weight or less frictional drag or both. You stop the centrifuge when the different molecules have traveled different distances down the tube and then remove the solution by puncturing the bottom of the tube or pumping the solution off t he top.
Today there are many other methods also used, as listed below.
When proteins and other macromolecules are treated with SDS, a strong detergent, they are denatured. They also become negatively charged because of the charge on the detergent, and the amount of detergent bound is so large that any differences in n ative charge are swamped. The bigger the macromolecule, the more SDS is bound, so that all macromolecules treated with SDS have the same ratio of charge to mass. So for molecules treated with SDS, the pull per u nit mass in an electric field is the same, and all molecules should have the same velocity IF there is no frictional drag. But electrophoresis with SDS is always done in a gel, so that the molecules must be pulled though the pores.