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I am a high school student and I want to know that how centrifugation technique really separates substances according to their molecular masses? In many books and sites, it is written that it is because of the centrifugal force which is much higher on particles of high molecular mass than on lower ones but in reality I know that there is no centrifugal force, and I am not able to think intuitively how all things are happening, please give me an insight?

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  • $\begingroup$ This would be an advanced topic to learn in high school, but it's worth pointing out that centrifugal forces do exist in rotating reference frames. So in the reference frame of a particle going in a circle, there is a centrifugal force pushing it away from the center, creating a buoyant force that separates more and less dense materials. $\endgroup$
    – Mia
    Jun 3, 2021 at 19:28
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    $\begingroup$ @Mia I challenge the concept of 'exists in a rotating reference frame'. I have two classes of reasons to dismiss the concept of 'exists in a rotating reference frame'. Class 1: it is a purported explanation that is more complicated than the thing it purports to explain. Class 2: Both in newtonian mechanics and in GR the concept of 'exists in a rotating reference frame' is redundant. In terms of GR: leave out the concept of 'exists in a rotating reference frame' and GR is still just as powerful. It may be that in your perception it's not redundant. That can be discussed. $\endgroup$
    – Cleonis
    Jun 3, 2021 at 20:53
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    $\begingroup$ Re class 1: It's not more complicated in all cases. There are plenty of situations in classical mechanics where using a rotating reference frame is a more elegant way to capture the dynamics. Re class 2: I don't see what the point of nitpicking the definition of "exists" is. It's an observable phenomenon. $\endgroup$
    – Mia
    Jun 3, 2021 at 21:42
  • $\begingroup$ @Mia As to observable phenomenon: let me make a comparison: Einstein 1905 discussed the magnet-and-coil observation, and that a Lorentz Aether theory gives two distinct explanations for the induced current, depending on whether the coil or the magnet is assumed to be stationary. Observed induced current is the same either way. The distinction made in terms of a Lorentz Aether theory is not inherent in the phenomena. Lesson learned: to have two distinct explanations for a single observation is not a good idea. Single explanation: we infer the existence of inertia. $\endgroup$
    – Cleonis
    Jun 4, 2021 at 7:29
  • $\begingroup$ @Mia I endorse the stackexchange policy that the comment section is not for protracted discussion. I'm eager to discuss this subject, but that has to happen somewhere else; the comment section is not the place for it. $\endgroup$
    – Cleonis
    Jun 4, 2021 at 7:30

2 Answers 2

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Start with visualizing a centrifuge for drying clothes.

To sustain circumnavigating motion a centripetal force must be provided. In the case of a clothes centrifuge the walls of the drum provide the required centripetal force.

There is a fairly strong adhesion between water and the fibers of the clothes. (I'm talking natural fibers here; cotton, linnen, wool, none of that plastic rubbish.) If you hang a soaking wet piece of textile on a clothesline some of the water will drip out, but that dripping stops when the textile is still pretty wet. In other words: pulling 1 G of acceleration does very little.

When the clothes are in the centrifuge drum, and the drum is spinning fast: for the water it is now up to the forces of adhesion to provide the required centripetal force. Given the radius of the centrifuge drum and the rate of rotation you can work out how many G's the centrifuge is pulling.

The walls of the centrifuge drum are perforated, so the water can exit the drum.


Generally when people say something like: 'so-and-so is being forced outward by centrifugal force' what they are referring is a situation where not enough centripetal force is provided to sustain a particular radius of circumnavigating motion.

A clothes centrifuge: the faster it is spinning the stronger the required centripetal force. When the required centripetal force exceeds the adhesion force the water migrates away from the center of rotation.

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  • $\begingroup$ Ok! I understood it, so, what actually exists is inertia and when we say that centrifugal force is pulling it outwards ,in inertial frame it means that not enough centripetal force is provided and actually it sustain the position (if it was initially at rest) but from the rotating frame say a centrifuge machine it appears to go radially outward w.r.t the test tube. $\endgroup$ Jun 4, 2021 at 5:55
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A centrifuge separates dense molecules from less dense molecules in the following manner.

1.The centrifuge turns with a specific amount of centrifugal force, say "y". 2. Heavier masses end up at the furthest point (or area) away from the axis of rotation (perpendicular to the floor and in centre of centrifuge)

No.2 occurs because heavier masses require a higher centripetal force. Here is the mathematical reason for it.

You see, centripetal force is given by m * centripetal acceleration. The only way to increase the centripetal force is to increase mass (centripetal acceleration is a constant value provided by the power level of the centrifuge).

Therefore, the heavier the mass = the greater centripetal force needed. Now we also know that centripetal force is the force that pulls something inward in a centrifuge.

However the "y" centripetal force is not enough to provide for the centripetal force required by heavier mass. Therefore, there is not enough inward pull for the heavier mass and so, the heavier mass stays the furthest away in the test tube from the axis of rotation. This is how centrifuges work in separating substances of different densities.

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