Expert: Expert - 3/17/2007

Question
QUESTION: Suppose a man pushes a block on a rough floor horizontally with a constant force.The work done BY THE MAN is ONLY dependent on the force HE EXERTS (and the displacement of course), it is COMPLETELY INDEPENDANT of the force of friction; which is the force he is overcoming, or any other force acting on the block for that matter.

By precisely the same argument ,how can the work done BY THE WATER ( i stress on "by the water" again) be in anyway related to the force it is overcoming,i.e. the atmospheric pressure?

About the solid question:When a metal kept in atmosphere is heated , it undergoes volumetric expansion.
It is said that during this expansion, the work done by the metal = (atmospheric pressure)*(change in volume)
So why atmospheric pressure?
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The text above is a follow-up to ...

-----Question-----
When water exposed to atmosphere is boiled ,why is the work done BY THE WATER taken as  (atmospheric pressure)*(Change in its volume)? Why atmospheric pressure? Should not the pressure taken here be the pressure developed inside the liquid?

And why may this be, if at all, equal to the atmospheric pressure?

I have seen these same calculations being done for a solid which expanding due to heating.Is this valid?
-----Answer-----
When a liquid expands against an atmosphere, the force it must overcome to do so is the atmospheric pressure, not its own pressure. What the liquid's pressure may be is irrelevant; what does matter is the force it must overcome -- ie, the atmospheric pressure.

I don't understand the question about the solid. Sorry.
ANSWER: > The work done BY THE MAN is ONLY dependent on the force HE EXERTS
> it is COMPLETELY INDEPENDANT[sic] of the force of friction

This is where you are going wrong. The above statement is false as far as PHYSICS is concerned.

Let's imagine a heavy weight one meter above a floor. It has a certain potential energy relative to the floor. As long as that weight stays at that height, its potential energy does not change. The only way to change its potential energy is to input energy into it. The energy input into this weight, that results in a change of the energy of the weight, is called "work" in the PHYSICS sense. If there is no change in the energy of the weight, there is no work done in the PHYSICS sense.

If I push and I shove to try to change the height of the weight, but fail to do so, I have not changed its energy. Thus, in the PHYSICS sense, I have done no work. I may have burned up a lot of calories, I may have strained myself terribly, I may even be able to truthfully say, "It was a lot of work to try to lift that thing" -- but the fact remains, I have imparted no energy to the weight. And thus, in the PHYSICS sense, I have done no work.

If you want to create a new type of science, where "work" means something different than what it means in physics, feel free. Just be aware that you will immediately run into the problem you've raised -- your use of the word "work" in a non-physics sense carries over to a use in a physics sense.

---------- FOLLOW-UP ----------

QUESTION: It seems you completely misread/misinterpreted (or both) my question.
If you would read my 1st follow on again ,even though i have capitalised "ONLY" i HAVE MENTIONED IN BRACKETS that the work is dependant on "the displacement of course".

My doubts are specifically about the water expantion problem.
The water molecules ARE displacing the air molecules ,but why should the force with which they 'push' the air molecules be equal to the atmospheric pressure?

Definition of work in the physics sense is very clear to me.

Answer
Alright, let's start over. We have water. It is expanding against air. The air fights back with a force of air pressure, the water overcomes this force, and pushes the air a certain distance.

Definition of "work"

Work expended to move an object = Force Overcome X Distance Moved

If you don't understand the above statement, I can't help you any further. I agree that the force overcome might be the friction force of an object sliding along a horizontal floor. Thus, even though no energy is given to that object moved, a force was overcome and thus work was expended -- all of which went to friction heat. But the above definition holds in this case as well.

> Definition of work in the physics sense is very clear to me.

If it is, then you must know that the following statement of yours is false.

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The work done BY THE MAN is ONLY dependent on the force HE EXERTS (and the displacement of course), it is COMPLETELY INDEPENDANT of the force of friction; which is the force he is overcoming, or any other force acting on the block for that matter.
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