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Homework Help: The friction force comes from where?

  1. Feb 23, 2010 #1
    1. The problem statement, all variables and given/known data
    We always encounter this expression: "Friction always opposes the motion". But what does it mean exactly? For example when we push a heavy box and it does not move we consider the friction opposite to the applied force. The second case when the box moves. In this case if we show the displacement by vector X the friction will be a vector opposite to X. Now consider a mass on an rotating horizontal disc. In this case we consider the friction equal to mrw^2 which w is the angular velocity and we consider the friction towards the center of rotation. How can we interpret the expression "friction is opposite to the motion". In the first case we dont have any motion at all. At the third case the displacement vector is different to the friction.
    What is the quantitative meaning of "motion"? Can we consider it the displacement vector?
    Can you explain this?

    2. Relevant equations

    3. The attempt at a solution
    Last edited: Feb 23, 2010
  2. jcsd
  3. Feb 23, 2010 #2
    I don't quite see it the way you do. The microscopic picture of friction involves the formation of temporary bonds b/w atoms of surfaces in contact. You see such a large surface in contact but actually due to the irregularities the actual area of contact is a small fraction of it. High pressure at these points leads to these bonds, something known as cold welding. Due to these electrostatic interactions RELATIVE motion is opposed, in whatever direction force acts on the object.
    And friction is a real force.
  4. Feb 23, 2010 #3
    ok i accept. But you did not answer the question. What is the direction of the friction with respect to the motion. Suppose that the motion is shown with a vector. For example vector X for displacement. Can you show the direction of the friction force?
  5. Feb 23, 2010 #4
    No I can't.
    Friction's direction w.r.t actual motion is not always the same. It only opposes relative motion. For example, a block on an accelerating body experiences friction in direction of its actual displacement but a little thought shows that had friction not been there it would have gone backwards - relative motion b/w the body and block.
    Similarly in your example of rotating frames, if you observe w.r.t the rotating frame there is a centrifugal force away from center so friction is radial. In this case too dispalcement vector and friction have no relation.
  6. Feb 23, 2010 #5
    To add on, there are actually three key types of friction between surfaces: static friction, kinetic friction, and rolling friction. The classic cases of friction involve the first two types of friction.

    Kinetic friction is the force that opposes the relative motion between two surfaces that are in contact and moving relative to each other. Static friction is the force that opposes the relative tendency motion between two surfaces that are in contact but not moving relative to each other.

    Static friction adopts a range of values from zero to a maximum value denoted the "limiting friction". It is dependent on the force that is attempting to cause the surfaces to move relative to each other (but no motion occurs yet). Kinetic friction, on the other hand, has an approximately constant value given the two surfaces. The value of the kinetic friction is usually a little less than the limiting friction.
  7. Feb 23, 2010 #6
    Thanks. We reached the good point now.
    Consider a rotating disc with a mass on it without motion with respect to the disc. As you mentioned above this a case of static friction with the limitation equals to μN. Now the angular velocity of the disc (W) begins to increase. The maximum angular velocity which the mass can stay without any motion with respect to the disc is when mrW^2 = μN. After this velocity the mass will move relative to the disc. Suppose we increase the velocity of the disc a bit more than this threshold velocity. Is it possible to analysis the movement of the mass, for example determine the path it moves.
  8. Feb 23, 2010 #7
    i believe with the law of inertia of newton, there will be a centrifugal force exists, hence the mass will try to move away from the centre of rotation, with the increase in the angular velocity of the disc, such force will increase as well.

    hence, the motion of the mass will be more likely to be same as the motion of escaping velocity, see the links

    of course if the gain in centrifugal force < limiting friction, the mass will only circulate in the disc

    however, this is just my thinking, need someone to clarify it
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