I want to clear my doubts (friction and work)

[shalini_monalisa]

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.

 

[Doc Al]

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?

 

[shalini_monalisa]

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.

 

[Doc Al]

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.

 

[Nidum]

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 ?

 

[StanEvans]

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.

 

[Khashishi]

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.

 

[StanEvans]

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.

 

[Doc Al]

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.

However the distance has a positive value, while the displacement does not.

The work done is the integral of force*displacement over the path ($\int \vec{F}\cdot d\vec{s}$).

 

[person_random_normal]

State the pencil thing clearly