In a tube of blood that is centrifuged, substances are in order by weight that the heaviest is at the bottom. Since the centrifugal force is applied outwards the center, why do things that have the bigger mass move towards the bottom of the tube while they have a lesser acceleration according to F=ma considering F is constant in this instance. What am I missing here ?

Question

In a tube of blood that is centrifuged, substances are in order by weight that the heaviest is at the bottom. Since the centrifugal force is applied outwards the center,  why do things that have the bigger mass move towards the bottom of the tube while they have a lesser acceleration according to F=ma considering F is constant in this instance.  What am I missing here ?

anonymous · Answer

What forces are acting on the blood contents?  Draw yourself a force diagram.

anonymous · Answer

Is it that since the centrifugal force is considered fictitious, only the centripetal force is valid for this calculation?

anonymous · Answer

Centrifugal force is fictitious, yes, but it is still useful.  Look at the problem in terms of centrifugal force.  The centrifuge spins with a constant angular velocity, so:$$F _{c}=m \omega ^{2}r$$Now consider what that force will be on different particles as a function of mass.

anonymous · Answer

Hmm, so F is not constant for each component at all. And there is a positive feedback between m and r am I correct?

anonymous · Answer

It's not really a feedback, but rather that force varies directly with mass and with radius.  To be a feedback either mass would have to be a function of radius or radius would have to be a function of mass.  You've got the general idea, though: the larger mass objects will experience greater force.

anonymous · Answer

Thanks Psi you really are a help.

anonymous · Answer

No problem.