# What exactly causes particles to drift toward the bottom of a tube in a centrifuge?

As I understand, centrifuges can effectively create gravitational fields that are a thousand times stronger than the Earth's. And it is this pseudo-gravitational force that causes particles to sink toward the bottom of a test tube, but this gravitational force within the tube is nothing but the centrifugal force, which is a fictitious force that appears only when we are working within the rotating reference frame.

So by which mechanism exactly do the particles sink toward the bottom of the tube? And how can we have a fictitious force, yet with real effects (the sinking of the particles)?

• Just because we call something a fictitious, or pseudo force, does not mean it does not exist and can be ignored. Dec 8 '19 at 0:19
• @AdrianHoward The Oxford English Dictionary defines fictitious as not real or true. Dec 8 '19 at 6:58
• This is due to the inertia of the particles.
– user238497
Dec 8 '19 at 7:18
• @Hilbert OK, then, more accurately call it an inertial force, and see if it is still not real or true Dec 8 '19 at 8:14
• @The Last Airbender Would you mind elaborating a litle more? Dec 8 '19 at 10:18

## 1 Answer

Rotating reference frame

In the rotating reference frame the centrifugal force explains the sinking of dense particles, just as gravity does on the surface of earth. Unfortunately, the terminology “fictitious” makes it seem as though it cannot do anything. For that reason I prefer the term “inertial force” instead of “fictitious force”.

In a non-inertial frame fictitious forces (or inertial forces) are necessary to explain the motion of objects and the stresses, strains, and other physical effects. Therefore, it is perfectly valid to attribute the settling to the centrifugal force.

Inertial reference frame

In the inertial reference frame it is important to note that the particles do not accelerate towards the bottom of the tube. Their acceleration is at all times towards the center. It is not the particles which sink outwards but rather the vial and the less dense materials are accelerated inwards faster.

• "It is not the particles which sink outwards" But the particles effectively reach the bottom of the vial. How can they be not outwardly accelerated, yet reach the end of the tube? Dec 8 '19 at 19:38
• @Hilbert the particles reach the end of the tube by being accelerated inwards less than the tube.
– Dale
Dec 8 '19 at 20:18