Conservative force vs nonConservative force

[jack1234]

In a given displacement of a particle, its kinetic energy increases by 25 J while its potential energy decreases by 10 J. Determine the work of the nonconservative forces acting on the particle during this displacement.
a. –15 J b. +35 J c. +15 J d. –35 J e. +55 J

workdone by non-conservative force
=change in mechanical energy
=25-10
=15

Hence I expect the answer to be c, but it turns up to be a, what is the reason?

 

[learningphysics]

I agree with you. It should be c) +15J.

 

[CaptainADHD]

The answer is -15 joules because the non-conservative force (I'm assuming friction) was acting opposite the direction of movement.

Friction is fighting whatever force is moving that object, and friction is losing. The loser in a force fight does negative work.

Think of it like this: Darth Vader always does positive work, because he always wins every fight. Whoever fights Vader does negative work, because Vader PWNS THEM every time.