Is Static Friction Capable of Doing Work?

[Doc Al]

I am not sure if the work done can be a negative value?
In thermaldynamics, the work done is always positive.

Of course work can be negative and that's true in thermo as well!

 

[russ_watters]

This new example is clearer to explain the work done I think.

No, it isn't. It isn't even remotely similar to the other scenarios and does not discuss the same concept. You are now asking if work can be negative, which has nothing to do with whether static friction can cause work to be done.

In any case, the answer to the new question is yes, work can be negative. Work (energy) can either go into or out of a system. Work in is positive, work out is negative. Looking at the net work equation , if the work out is greater than the work in, the net work is negative. http://en.wikipedia.org/wiki/Work_(thermodynamics)#Mathematical_definition

 

[pixel01]

In the spring example, at one time, the spring pulls the mass, and the connection does a work. at other time, the mass pulls the spring, the connection should then consume work or does negative work? If we accept that the knot can create work?

 

[pixel01]

Of course work can be negative and that's true in thermo as well!

OK, I should check this first.

 

[russ_watters]

In the spring example, at one time, the spring pulls the mass, and the connection does a work. at other time, the mass pulls the spring, the connection should then consume work or does negative work? If we accept that the knot can create work?

You mean the connection between the mass and the spring? Yes, if you didn't know the spring was there, you could say that the knot did work on the mass (+), then the mass did work on the knot (-). [incidentally, if you ignore the spring, now the scenario is pretty much identical to the previous scenario of a string pulling a mass]

 

[russ_watters]

Try this, pixel01 - draw a picture of the system, then draw a circle around the part you want to analyze. For any of these scenarios, knowing how to properly define the system is the key. For the stacked-boxes example, you would draw your circle around the ground, the entire lower box, and part of the upper box - but not the person's finger. That way the only force you are looking at is the force between the boxes and the only motion is between the lower box and the ground.

 

[pixel01]

Right, the work can be negative and the spring example is meaningless.

Anyway, I am not convinced at the statement: static friction (or string, bolts or nuts...) can do work.

 

[miss photon]

i think we can say that static friction is capable of doing work in certain frames of reference.

 

[russ_watters]

Not sure how to help you - have you tried drawing a picture?

 

[arildno]

Right, the work can be negative and the spring example is meaningless.

Anyway, I am not convinced at the statement: static friction (or string, bolts or nuts...) can do work.

Why not?

You have some type of misconception here.