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# centrifuge

Discussion in 'MCAT Study Question Q&A' started by Aria88997, May 5, 2012.

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1. ### Aria88997Established Member

Joined:
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Messages:
348

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Hi all,

Can anyone please explain to me how a centrifuge exactly speeds up the sedimentation process. I understand that under noncentrifuge conditions the two forces acting on the particle in the test tube is the force of gravity and the force of buoyancy. Obviously if the test tube laid there for ages then the sedimentation would take place as the force of gravity is slightly stronger in magnitude than the force of buoyancy. However, what makes a centrifuge speed this process up? I understand that in the centrifuge there is an extra centripetal force towards the rotation axis, but how does this force cause the particles to settle more quickly?

Thanks in advance.
2. ### dmf2682New Member

Joined:
May 29, 2011
Messages:
1,310
Status:
Pre-Medical
when stuff goes round a turn the more dense material goes to a higher radius, the less dense material goes to a lower radius. That's how the separation works.

to prove it, you'll need a helium balloon, a muddy field, and an automobile.

Take a helium balloon and place it in the automobile. Then drive the automobile out to the field. Now, and here's the fun part: Accelerate and make a turn to the left. As the automobile keeps turning you'll notice that while you (as a dense object) will feel thrown to the right (higher radius), but the balloon will interestingly move to the left (lower radius). When you've made your observation, quickly accelerate while making an even harder turn to the left. If your automobile is powerful enough you'll make a donut.

I guess you don't need to make the donut, but it's fun to try.
3. ### johnwanderingNew Member

Joined:
May 18, 2009
Messages:
418
I used to think about this because Centripetal Force is force inward, what force creates sedimentation outward?

Basically, the Centripetal Force that holds a centrifuge together ONLY acts on the "centrifuge buckets" and not the contained liquid.
The liquid wants to continue moving straight in a linear velocity, but the buckets "impede their path."

Hence, the buckets are causing an acceleration of the liquid by causing it "turn." This acceleration is the force that causes the sedimentation in a centrifuge.
4. ### Aria88997Established Member

Joined:
May 6, 2009
Messages:
348
i have been trying to make free body diagrams and put this into the formulas, but i still dont have a grasp on it for some reason

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