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Frictional force

  1. Jun 5, 2015 #1
    Does the static friction get converted into kinetic friction while moving
     
  2. jcsd
  3. Jun 5, 2015 #2

    Simon Bridge

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    No.

    Friction is an emergent property of underlying intermolecular forces - it's an "effect" of something else.
    So one kind of friction does not convert to another kind - better to think this way:

    kinetic friction takes over from static friction at the instant the object starts moving.

    For an applied force ##F##, with static and kinetic friction ##f_s## and ##f_k## respectively, we can find the acceleration as:
    $$a = \begin{cases}
    0 &: F<f_s\\
    (F-f_k)/m &: F\geq f_s \end{cases}$$

    [edited to tidy up the notation]
     
    Last edited: Jun 5, 2015
  4. Jun 5, 2015 #3
    At the instance when f_k take over f_s does f_s becomes zero
     
  5. Jun 5, 2015 #4

    jbriggs444

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    I does not become zero so much as it becomes irrelevant. When there is relative motion, f_s does not apply -- the frictional force between the surfaces is given by f_k. When there is no relative motion, f_k does not apply -- the frictional force between the surfaces is limited by f_s.
     
  6. Jun 5, 2015 #5
    See this
     
  7. Jun 5, 2015 #6
    Thank you very much
     
  8. Jun 5, 2015 #7

    Simon Bridge

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    Note: if ##f## (no subscript) is the friction force, then newtons law says ##F-f=ma## where

    ##f=\begin{cases} f_k &: F > f_s\\ F &: F < f_s \end{cases}##

    What happens ##F=f_s## technically depends on the wording of the question - which is why I'm being deliberately ambiguous about where the "equality" goes.​
     
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