Static friction and work

arildno

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Caveat:
Pixel is defining his system in such a manner so that it is not apparent to him that static friction is doing work on the lower box.

That, however, doesn't make his system "wrong", merely unsuitable for analyzing the particular situation at hand.

pixel01

OK, I have another scenario. A spring is connected to a mass at one end and fixed at the other. Now, you make the system oscillate by one force. The system can oscillate for very long time. If we consider the connection can do work (it's similar to a string etc..), that means there's time the work is negative and sometimes the work is positive.
I am not sure if the work done can be a negative value?
In thermaldynamics, the work done is always positive.

arildno

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Why move to another scenario?
It is not at all obvious you have understood the previous one.

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pixel01

This new example is clearer to explain the work done I think. If the connection can do some work, we have to accept some negative work and that is impossible.

russ_watters

Mentor
Caveat:
Pixel is defining his system in such a manner so that it is not apparent to him that static friction is doing work on the lower box.

That, however, doesn't make his system "wrong", merely unsuitable for analyzing the particular situation at hand.
Yes, you are correct. In *his* box scenario, there is no motion between the boxes, therefore no work being done between the boxes. He's correct about that. But in *our* box scenario, there is motion between the box and the ground and therefore work being done on the lower box by the upper box with respect to the ground.

He's basically using a scenario where no work is being done to say that no work can be done, while we are showing a scenario where work is being done.

Doc Al

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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

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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

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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

Mentor
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

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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

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

arildno

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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.

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