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Confusion of centrifugal force

  1. Feb 9, 2015 #1
    Hi,

    I am kind of confused about how a centrifugal force works. Every where i search there seems to be 2 opinions, that centrifugal force is indeed a force and others that argue that it is not.

    What I want to understand is in (for example) a centrifuge, which rotates very quickly with a test tube to separate particles in a mixture (for example sand and water) and the sand move to the bottom of the test tube. my question is what moves them? There seems no force that actually "pushes" these sand particles to the bottom (except centrifugal force).

    Also, I think that from what I understand, the centripetal force provided in this example is by the friction force between the sand particles and the water particles. Am I missing any force here?

    Sorry for the long post :)
     
  2. jcsd
  3. Feb 9, 2015 #2
    According to the framework of Newton's laws an object moves in a straight line with constant velocity unless compelled to change its motion due to an outside force. To keep something moving in circular motion at constant speed the only force required is an inwardly directed force (centripetal force). Think of swinging something in a circle with a string - you need to pull inward.

    The so-called centrifugal force arises when attempting to analyze motion from within the rotating frame. If you attach your coordinate system to the swinging ball then F=ma wouldn't make correct predictions or explanations. There are three 'fictitious' forces that must be added to correct for what is observed in a rotating frame. The centrifugal force is one of these forces.

    In the laboratory frame of reference a centrifuge works because the inertia of the denser particles is greater than the lighter ones and so they end up on the 'bottom.' In the frame of the rotating system they end up on the bottom because there is a greater centrifugal force on them.
     
  4. Feb 9, 2015 #3

    A.T.

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    Here you are referring to the inertial (fictitious) centrifugal force, which exisits only in the rotating rest frame frame of tube. In the inertial reference frame of the lab there is no centrifugal force that accelerates the particles outwards. Instead, the bottom of the tube accelerates towards the center faster than the particles, so they meet.

    Also some buoyancy. But these provide less centripetal force, than needed to achieve the same centripetal acceleration as the tube.
     
  5. Feb 9, 2015 #4
    Ahaaaaaa.
    Thanks guys for explaining, this was going to be a long night indeed :D
     
  6. Feb 9, 2015 #5

    rcgldr

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    It's an argument over the definition of centrifugal force. To clarify the difference, wiki uses the term reactive centrifugal force to refer to the real force that is part of a Newton third law pair of forces. Say an object is moving in a circular motion due to a string where the other end if fixed at the center of the circular path of that object. The string exerts an inwards centripetal force on the object, and the object exerts an outwards force onto the string, a reaction to the centripetal acceleration, which some call reactive centrifugal force. Note that the centripetal force acts upon the object, while the "reactive centrifugal force" acts on the string. The net force on the object is a centripetal force, with a corresponding centripetal acceleration.

    Using this same example but from a rotating frame of reference that rotates at the same angular velocity as the object, and with it's center at the center of the circular path, again you have Newton third law pair of forces, an inwards force exerted by the string onto the object, and an outwards reactive force exerted by the object onto the string. However in the rotating frame of reference the object is not accelerating with respect to the rotating frame, so the net force on the object as observed from the rotating frame is zero. For a rotating frame of reference a fictitious centrifugal outwards force is exerted onto the object (not the string), countering the real inwards force exerted by the string onto the object, so in the rotating frame of reference there is zero net force on the object (with respect to the rotating frame). Note that this fictitious centrifugal force is not part of a Newton third law pair.
     
  7. Feb 9, 2015 #6

    A.T.

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    Or about whether inertial forces are "forces". Both arguments are semantic.
     
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