I want to clear my doubts (friction and work)

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    Doubts Work
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Discussion Overview

The discussion revolves around the concept of work done by friction, particularly in the context of a pencil moving in a circular motion on a page. Participants explore the implications of force application, movement, and the definitions of work in physics.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions why the work done by friction on a pencil is considered zero when the pencil moves in a circle, suggesting that both the tip of the pencil and the friction are in motion.
  • Another participant challenges the assumption that work done is zero, indicating that this conclusion is derived from a book and asking for clarification on the reasoning behind it.
  • It is suggested that the inference of no net movement of the point of application of force is incorrect, prompting a request for further explanation on why this is the case.
  • There is a discussion about whether work should be considered as a scalar or vector quantity, with implications on how this affects the understanding of work done in circular motion.
  • Some participants emphasize that moving in a circle constitutes movement, even if the starting and ending points are the same, and differentiate between distance and displacement in this context.
  • Mathematical expressions related to work done, such as the integral of force and displacement, are mentioned but not fully resolved in the discussion.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of work done by friction in circular motion, with no consensus reached on the underlying principles or definitions involved.

Contextual Notes

There are unresolved assumptions regarding the definitions of work, the nature of movement in circular motion, and the distinction between scalar and vector quantities in this context.

shalini_monalisa
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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.
 
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shalini_monalisa said:
then why is the work done by friction on pencil zero when pencil makes a circle on a page ?
Why do you think the work done is zero?
 
Doc Al said:
Why do you think the work done is zero?
I am not saying the work done by friction is zero. it is answered thus in the book.
 
shalini_monalisa said:
I am not saying the work done by friction is zero. it is answered thus in the book.
Can you give an exact reference? I'm curious as to what the book says.
 
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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:
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.
 
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.
 
Khashishi said:
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.
However the distance has a positive value, while the displacement does not.
 
StanEvans said:
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.
Work is a scalar quantity.

StanEvans said:
However the distance has a positive value, while the displacement does not.
The work done is the integral of force*displacement over the path (##\int \vec{F}\cdot d\vec{s}##).
 
  • #10
State the pencil thing clearly
 

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