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Homework Help: Coefficient Of Friction (Circular Motion)

  1. Jan 6, 2009 #1


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    This is the concept that I am understanding for coefficient of friction. Static coefficient of friction is one which refers to friction which is considered when the object is stationary relative to the other contact object. Kinetic coefficient of friction is one which refers to the friction which is considered when the object is in motion relative to the other contact object.

    But, I've come across many question in circular motion which talk about static coeffient of fricition even though it's asking about calculating maximum speed. If static coeffient of friction is when the object is stationary then why is it used to calculate speed, acceleration in circular motion (Surely, we consider coefficient of kinetic friction)? Does the concept differ in circular motion?
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  3. Jan 6, 2009 #2

    Doc Al

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    Kinetic friction only occurs when contact surfaces slip against each other--thus the surfaces are in relative motion. Sometimes an object as a whole can be moving, yet the contact surface will not move with respect to the ground. For example, a car can move while the part of the tire in contact with the ground is always at rest with respect to the ground. As long as there's no slipping (or skidding) the only friction you need to worry about is static friction.

    Feel free to give a specific problem that we can look at in detail.
  4. Jan 12, 2009 #3
    As for a specific problem, what about a ball bearing for example. I have a wheel rim attached to an axle via a ball bearing. How would I calculate the friction between the bearings? Is that Kinetic friction?
  5. Jan 12, 2009 #4


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    The ball bearing will roll on the axel and on the housing - no slipping so it is STATIC friction.
  6. Jan 12, 2009 #5
    Right. So if the wheel rim is free spinning (in the air), what are the forces acting against it? I now assume there is static friction between the housing and axle, I would also think there is some form of rolling resistance between these 2 components as well?
  7. Jan 13, 2009 #6
    Bearings are pretty different from the OP question. Ball and roller bearings have to be treated in terms of their rolling resistance, which is best treated as a separate kind of friction. See, for example,
    http://evolution.skf.com/zino.aspx?articleID=14959 [Broken]
    Last edited by a moderator: May 3, 2017
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