Enriching uranium is difficult because the isotopes are practically identical in chemistry and very similar in weight: U-235 is only 1.26% lighter than U-238 (note this applies only to uranium metal). Centrifuges need to work with a gas rather than a solid, and the gas used here is uranium hexafluoride. The relative mass difference between 235UF6 and 238UF6 is less than 0.86%. On the other hand, separation efficiency in a centrifuge depends on absolute mass difference. Separation of uranium isotopes requires a centrifuge that can spin at 1,500 revolutions per second (90,000RPM). If we assume a rotor diameter of 20 cm (as in some modern centrifuges), this would correspond to a centripetal acceleration of around 900,000 x g (around 42 times the max speed of a standard, lab benchtop microcentrifuge and between 0.9 to 9 times the max speed of a standard lab ultracentrifuge) or a linear speed of greater than Mach2 in air (Mach1 = sound velocity, in air ca. 340 m/s) and much more in UF6. For comparison, automatic washing machines operate at only about 12 to 25 revolutions per second (720–1500RPM) during the spin cycle, while turbines in automotive turbochargers can run up to around 2500–3333 revolutions per second (150,000–200,000RPM).How A Turbo WorksHowStuffWorks "How Turbochargers Work"
