# Centrifugal Force?Watch

#1
Suppose I put some insoluble dust in a test tube containing water. Then spin the test tube in a circle. Why does the dust collect at the outer end of the test tube.

I've been told that this is due to centrifugal force but some people also claim centrifugal force isn't real. Can anyone explain this without using centrifugal force?
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3 months ago
#2
(Original post by esrever)
Suppose I put some insoluble dust in a test tube containing water. Then spin the test tube in a circle. Why does the dust collect at the outer end of the test tube.

I've been told that this is due to centrifugal force but some people also claim centrifugal force isn't real. Can anyone explain this without using centrifugal force?
Hi. I currently do A level physics and we were told that centrifugal force does not exist unlike what we were told in GCSE. The dust particles (otherwise known as the pellet) gather at the bottom due to a centripetal force (centre-seeking force). This occurs because the force (such as gravity) always acts towards the centre of the circle with uniform acceleration. The speed will remain constant however velocity will always be changing as velocity is a vector quantity and the direction if always changing. Another example will be the moon orbiting the Earth. The moon is in orbit due to a centripetal force (in this case gravity) however the moon travels at a speed that is fast enough that the moon doesn't crash into the earth but slow enough that the moon doesn't fall out of orbit. Hope this helps
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#3
(Original post by xshannon_heslopx)
Hi. I currently do A level physics and we were told that centrifugal force does not exist unlike what we were told in GCSE. The dust particles (otherwise known as the pellet) gather at the bottom due to a centripetal force (centre-seeking force). This occurs because the force (such as gravity) always acts towards the centre of the circle with uniform acceleration. The speed will remain constant however velocity will always be changing as velocity is a vector quantity and the direction if always changing. Another example will be the moon orbiting the Earth. The moon is in orbit due to a centripetal force (in this case gravity) however the moon travels at a speed that is fast enough that the moon doesn't crash into the earth but slow enough that the moon doesn't fall out of orbit. Hope this helps
I mean centrifugal separation like this. Why do the dust particles collect at the end of the test tube? Why not somewhere else?

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3 months ago
#4
(Original post by esrever)
I mean centrifugal separation like this. Why do the dust particles collect at the end of the test tube? Why not somewhere else?

I believe it is because that is the direction of their velocity. If you imagine spinning around with a ball on a thread and letting go it won't hit you, but it will fly forward.

Unless.you mean at the bottom of the test tube, that is because of gravity.
Last edited by ZombieTheWolf; 3 months ago
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3 months ago
#5
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#6
(Original post by ZombieTheWolf)
I believe it is because that is the direction of their velocity. If you imagine spinning around with a ball on a thread and letting go it won't hit you, but it will fly forward.

Unless.you mean at the bottom of the test tube, that is because of gravity.
Yes I meant at the bottom of the tube. But it's not due to gravity as even if the tubes in the video were horizontal, same effect would be seen.

I suspect it's got something to do with heavier particles experiencing a much greater force than lighter particles at same angular velocity.
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3 months ago
#7
(Original post by esrever)
Yes I meant at the bottom of the tube. But it's not due to gravity as even if the tubes in the video were horizontal, same effect would be seen.

I suspect it's got something to do with heavier particles experiencing a much greater force than lighter particles at same angular velocity.
Sorry I cannot watch any of the youtube videos as they are not hyperlinked and I cannot copy the link.

Yes, F=ma.
Greater mass means greater force is experienced so you are very correct there.

Also, are you trying to ask why it isn't moving into the inside end of the tube despite centripetal force moving inward? Here is a really good extract from quora:

An object in uniform circular motion*is*falling in. That’s why it’s moving in a circle instead of a line.
Try this: when a car applies its brakes, there is a force*backwards. However the car does not move backwards when the brakes are applied! Rather, its velocity*changes*in this direction. This change is applied over time by the force.
Force does not make things begin moving in a certain direction. It makes the velocity*change*in that direction. An object moving in a circle is going to change velocity toward the center, not suddenly start moving toward the center.
Last edited by ZombieTheWolf; 3 months ago
1
3 months ago
#8
the dust particles obtain a greater mass than water and when a centrifuge is used a greater centripetal force is required to follow the circular path. This is what makes the dust particles move outwards to the bottom of the tube.
1
#9
(Original post by xshannon_heslopx)
the dust particles obtain a greater mass than water and when a centrifuge is used a greater centripetal force is required to follow the circular path. This is what makes the dust particles move outwards to the bottom of the tube.
But centripetal force is . Both force and mass are increased.
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3 months ago
#10
(Original post by esrever)
Suppose I put some insoluble dust in a test tube containing water. Then spin the test tube in a circle. Why does the dust collect at the outer end of the test tube.

I've been told that this is due to centrifugal force but some people also claim centrifugal force isn't real. Can anyone explain this without using centrifugal force?

It depends on what you mean by real.
Centripetal force is centrifugal force and centrifugal force is centripetal force.
We called centripetal force as a centrifugal force when the system that we want to analyze is viewed from a non-inertial frame of reference or accelerated frame of reference.

(Original post by esrever)
I mean centrifugal separation like this. Why do the dust particles collect at the end of the test tube? Why not somewhere else?

To use centripetal force to explain centrifugal separation, we need to identify what are the real force that make up the centripetal force in the centrifugal separation.

The forces that are responsible for the centripetal force in the centrifugal separation are:

buoyant force (Fb) and vicious force (Fd)

According to Newton’s 2nd law:

Fb + Fd = ma

Fb + Fd = m r ω2
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