# Is there a work done against friction in this case?

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#1
I’m required to find work done against friction in part(c). According to the answer, it is equal in magnitude to the work done by friction... but I feel it should be 0J since there’s no other force that can do work against friction. Am I correct?
Last edited by richie.harvy; 4 weeks ago
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4 weeks ago
#2
(Original post by richie.harvy)
I’m required to find work done against friction in part(c). According to the answer, it is equal in magnitude to the work done by friction... but I feel it should be 0J since there’s no other force that can do work against friction. Am I correct?
You are arguing about semantics. The work done against (the phenomenon of) friction is the same as the work done by (the force of) friction.
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#3
But there has to be a force to do work AGAINST friction no? There ain’t any such force here. Isn’t it?
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#4
(Original post by lordaxil)
You are arguing about semantics. The work done against (the phenomenon of) friction is the same as the work done by (the force of) friction.
Also, why is work done by friction negative and work done against positive?
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4 weeks ago
#5
(Original post by richie.harvy)
Also, why is work done by friction negative and work done against positive?
I didn't see the diagram in your original post - now it is clearer what you are asking.

The box has an initial momentum and so due to its inertia, it will continue to move unless acted on by a net force in horizontal direction. That force is the friction, which acts in a direction opposite to the direction of travel. The work done by friction is negative because it is removing energy from the box, until it comes to rest. The work done against friction is positive because it is contributing energy to extenal system which will mostly be dissipated as heat. The sign here is purely a matter of convention.
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