Question

14. "The work done by the frictional force on a pencil
in drawing a complete circle of radius
1/π metre on the surface by a pencil of
negligible mass with a normal pressing force
N = 5 N (u = 0.5) is :
(1) + 4J (2) -3J (3) - 2 J (4) - 5J
Here the answer should be zero as displacement is zero but it's not why??
please explain it....
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Answer 1

points to remind:-

1. friction is a non conservative force...that means its any work done is not reversible..its always get lost as heat .
2. so its not depend on initial and final position of the object....
3. instead it depends on the the path it travels throughout the motion...
4. so here when its path dependent then its work will be done on every point of the suface it moves....it convert into heat

and also...work is defined as

the result of force applied on an object (which is in the form of distance or displacement it undergoes)

☆ and this is becomes the actual displacement...when its a conservative force...eg gravity

you lift the object from the ground and again come back to ground ....W=0...

• as going up the Wmg is -ve as its opposite to the dirn of motion...
• and going down the Wmg is +ve as in the dirn of motion
• so net work is 0

☆ but in the friction at every point it acts in the opposite of motion of pencil and same direction of motion surface of paper..

• so for the pencil at every point it works opposite in dirn of motion of circle....whole net work is negative...
• there is no cancel out here as we see in gravitational force

hope it gets fit in your cerebrum...

Good luck ♧♧

Answer 2

Answer:

I don't know if you have any questions sorry better luck next time you get to the