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B I want to clear my doubts (friction and work)

  1. Sep 27, 2016 #1
    i read in a book-'work is done when a force moves its point of application.this involves movement of the object on which force acts and movement of the force itself.'
    when this is true ,then why is the work done by friction on pencil zero when pencil makes a circle on a page ?
    i assume that tip of pencil (point of app.of force) and friction both are moving.please clarify where i am going wrong.
     
  2. jcsd
  3. Sep 27, 2016 #2

    Doc Al

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    Why do you think the work done is zero?
     
  4. Sep 29, 2016 #3
    I am not saying the work done by friction is zero. it is answered thus in the book.
     
  5. Sep 29, 2016 #4

    Doc Al

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    Can you give an exact reference? I'm curious as to what the book says.
     
  6. Sep 29, 2016 #5

    Nidum

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    You may be confused about the true meaning of ' when a force moves it's point of application ' .

    When drawing a circle the point of application of the pencil force appears to end up back where it started and it is easy to infer from this that no net movement of the point of application of the force has taken place and therefore no work has been done .

    That inference is completely wrong though . Can you tell us why ?
     
    Last edited: Sep 29, 2016
  7. Sep 29, 2016 #6
    Is there an assumption of the work done being vector or scalar, as if it is scalar then this may infer that work has been done, while vector may not have a net work done.
     
  8. Sep 29, 2016 #7
    Please use correct capitalization and punctuation in your questions.
    Moving in a circle is still moving, even if you end up in the same place.
     
  9. Oct 1, 2016 #8
    However the distance has a positive value, while the displacement does not.
     
  10. Oct 1, 2016 #9

    Doc Al

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    Work is a scalar quantity.

    The work done is the integral of force*displacement over the path (##\int \vec{F}\cdot d\vec{s}##).
     
  11. Oct 1, 2016 #10
    State the pencil thing clearly
     
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