Why work is force times distance?

[Chaitanya Gaur]

So, here we are trying our best to describe physical universe. We start from displacement, time, velocity and then force, but I don't get why would we define a physical quantity called 'work' as force times distance?

Is it just some quantity we defined in physics because it turns out to be useful and it doesn't have much of reference with idea of 'work' in usual meaning itself?

 

[phinds]

Is it just some quantity we defined in physics because it turns out to be useful and it doesn't have much of reference with idea of 'work' in usual meaning itself?

Yes, It is often the case that science uses common words in different or more restrictive or differently defined ways than regular English does. Get used to it.

 

[Mark44]

why would we define a physical quantity called 'work' as force times distance?

Why not? If your car runs out of gas and you have to push it 50 m. to a gas station you are doing something -- let's call that work. If you have to push it twice as far, you are doing twice the work. Or if you have to push a car of only half the mass for 50 m., you are doing only half the work.

Strictly speaking, work isn't force times distance. This is true only if the force applied is constant and in the same direction as the object is moving. If the force varies or is applied at a varying angle, then the formula is more complication.

 

[FactChecker]

Is it just some quantity we defined in physics because it turns out to be useful and it doesn't have much of reference with idea of 'work' in usual meaning itself?

If you consider 4 cases, I think you will agree that the definition agrees well with our concept of physical work:
1) Push an object a short distance and each inch is easy: little work
2) Push an object a great distance and each inch is hard: a lot of work
3) Push an object a short distance and each inch is hard: a medium amount of work
4) Push an object a great distance and each inch is easy: a medium amount of work

PS. I see from the following post by @stevendaryl that the question may be much more profound than I thought. Quantaties that are conserved can have be very profound significance.

 

[stevendaryl]

So, here we are trying our best to describe physical universe. We start from displacement, time, velocity and then force, but I don't get why would we define a physical quantity called 'work' as force times distance?

Is it just some quantity we defined in physics because it turns out to be useful and it doesn't have much of reference with idea of 'work' in usual meaning itself?

The reason that work is important in physics is because of the conservation of energy. If you perform work on a system, the energy you expend must go into either increasing the kinetic energy, or increasing the potential energy, or increasing the thermal energy (heating up the system), or maybe some other forms of energy.

If there is no potential energy or thermal energy that is changing, then this becomes:

$W = \Delta KE$

 

[Grinkle]

it doesn't have much of reference with idea of 'work' in usual meaning itself?

You will get into difficulty if you want to argue that one vs another English word should have been chosen to classify a defined scientific thing. Your question about what makes force times displacement worth considering as a 'thing' is to me a great question.

Why call it "work" is to me a red herring. In any case, welcome to PF. :-)

 

[Aufbauwerk 2045]

Of course force, energy, and work can be confusing concepts. Here's a somewhat related question which I've even heard discussed on radio. I'm curious how people would explain this one to a science student in primary or secondary school.

We know that gravity acts on a refrigerator magnet. There is a downward force on the magnet, yet it does not fall. Will the magnet ever "run out" of magnetism, and if so, is this because it is expending energy to keep itself attached to the refrigerator? If it is in fact expending energy, why do we say it's not doing any "work" to stay attached?

After all, as my version of the question goes, if I am a first-grader standing near the top of a hill, pulling on my red wagon, where my little sister happens to be sitting, just enough to keep it from moving so it won't run downhill, because I'm too weak to pull it any farther uphill, then I am doing "work" as we normally understand it. My muscles certainly ache after holding on to that wagon until my dad comes along and takes over. I am applying a force to the wagon, I am expending energy to keep it from moving, my energy is running low, yet physics says I am only doing "work" when I pull the wagon up the hill. Why?

 

[phinds]

After all, as my version of the question goes, if I am a first-grader standing near the top of a hill, pulling on my red wagon, where my little sister happens to be sitting, just enough to keep it from moving so it won't run downhill, because I'm too weak to pull it any farther uphill, then I am doing "work" as we normally understand it. My muscles certainly ache after holding on to that wagon until my dad comes along and takes over. I am applying a force to the wagon, I am expending energy to keep it from moving, my energy is running low, yet physics says I am only doing "work" when I pull the wagon up the hill. Why?

Because as has already been explained several times in this thread English and science do not use words the same way.

 

[CWatters]

My guess is that the first example of people understanding the concept that work = force * distance came from ploughing a field.

 

[Aufbauwerk 2045]

Because as has already been explained several times in this thread English and science do not use words the same way.

It depends what we mean by "explain." Some students will not be satisfied with that answer. Or they may think physics uses words in a sloppy or confusing fashion. Is there an alternative word that can be used instead of "work" in such cases? Clearly something is going on, so what do we call it?

How would a teacher explain this, other than just saying "that's how we define 'work', no reason." I can hear the students after class saying, "that teacher really didn't explain it." Why do we use the word the way we do? Maybe it comes out of the development of mechanics theory as applied to machines? Maybe someone said a machine is not doing work if nothing is moving? I don't have a good answer, so I am curious about this from a history of science or teaching standpoint if nothing else.

 

[phinds]

It depends what we mean by "explain." Some students will not be satisfied with that answer. Or they may think physics uses words in a sloppy or confusing fashion. Is there an alternative word that can be used instead of "work" in such cases? Clearly something is going on, so what do we call it?

How would a teacher explain this, other than just saying "that's just how we define 'work', no reason." I can just hear the students after class saying, "that teacher really didn't explain it." Why do we use the word the way we do? Maybe it comes out of the development of mechanics theory as applied to machines? Maybe someone said a machine is not doing work if nothing is moving? I don't have a good answer, so I am curious about this from a history of science or teaching standpoint if nothing else.

Well, reread post #5

 

[jbriggs444]

Is there an alternative word that can be used instead of "work" in such cases? Clearly something is going on, so what do we call it?

We could call it "grobnitz". But that lacks the hook to experience that "work" possesses.

 

[Aufbauwerk 2045]

We could call it "grobnitz". But that lacks the hook to experience that "work" possesses.

You may well be on to something.

Maybe there is a really long compound German word that means something like

"effectthatkeepsmagnetfromfallingofftherefrigeratorbutisnotdoingwork."

Speaking of work, I must go now. :)

 

[Vanadium 50]

Some students will not be satisfied with that answer.

Well, the physicists of the world are not going to change their language because some freshman doesn't like it.

 

[PeterDonis]

Will the magnet ever "run out" of magnetism, and if so, is this because it is expending energy to keep itself attached to the refrigerator?

This is an easy one: no.

My muscles certainly ache after holding on to that wagon until my dad comes along and takes over. I am applying a force to the wagon, I am expending energy to keep it from moving, my energy is running low, yet physics says I am only doing "work" when I pull the wagon up the hill. Why?

Because the wagon isn't moving. The only reason you're expending energy is that the human body is not designed to just assume a static position and stay there. Your muscles and bones can't just lock into place; you have to continually shift them around just to keep pushing on the wagon. But that's a particular property of the human body, not a general physical property of anything that could possibly hold the wagon motionless. So your confusion here is because you're focusing on an irrelevant feature of the situation as you've described it.

To remove the irrelevant feature, chock the wagon's wheels so they can't roll, and stop trying to push on it and walk way. Then the wagon can sit there motionless indefinitely without anything expending any energy at all (just as the magnet can stay stuck to your refrigerator indefinitely without anything expending any energy at all). Which is why physicists say no work is being done.

 

[bhobba]

See the work-energy theorem - the change in a particles kinetic energy between points x1 and x2 equals the work done on the particle between x1 and x2. So for the theorem to apply you must define work in the usual way.

You can find a discussion on it in - Classical Mechanics - David Morin - Chapter 5. Noether's Theorem determines the energy of a free particle as its Kinetic Energy by the definition of energy as the conserved quantity related to time translation invariance and the work energy theorem determines why its useful to then define this thing called work. Noether etc is discussed in Chapter 6 of the same book.

Strangely I couldn't find it in Landau which was interesting - it may be there but tucked away as a problem or something like that. However Morin was written to be more sophisticated than your usual first year classical mechanics treatment for use at Harvard, and the author writes it could even be used at High School where he thinks it will be - what were his words - a hoot. I agree - but you need to have done calculus which of course you would expect virtually everyone to have done going to a school like Harvard.

Thanks
Bill

 

[bhobba]

Of course force, energy, and work can be confusing concepts.

Only if you do not know Noether and the Lagrangian formulation (which of course you need to understand Noether) - then its a snap. That's why I think it should be taught as early as possible - but you do need calculus after which a book like Morin's is accessible for motivated students even at High School.

Thanks
Bill

 

[Aufbauwerk 2045]

Only if you do not know Noether and the Lagrangian formulation (which of course you need to understand Noether) - then its a snap. That's why I think it should be taught as early as possible - but you do need calculus after which a book like Morin's is accessible for motivated students even at High School.

Thanks
Bill

Actually I was thinking of a simpler explanation! But anyway, thanks for the suggestions. I had in mind what I believe Feynman said about explaining something to a 10 year old.

I guess you people are right. I will just say it's how work is defined, then encourage people to learn more about physics if they want a deeper understanding. Same for force and energy. However, if I come across an explanation a ten year old can understand, I will pass it along.

 

[Aufbauwerk 2045]

If you consider 4 cases, I think you will agree that the definition agrees well with our concept of physical work:
1) Push an object a short distance and each inch is easy: little work
2) Push an object a great distance and each inch is hard: a lot of work
3) Push an object a short distance and each inch is hard: a medium amount of work
4) Push an object a great distance and each inch is easy: a medium amount of work

PS. I see from the following post by @stevendaryl that the question may be much more profound than I thought. Quantaties that are conserved can have be very profound significance.

I like your 4-point answer in any case.

 

[russ_watters]

Of course force, energy, and work can be confusing concepts.

I don't see why. Work is force times distance, which is the mechanical expending of energy. And force is just what a scale reads.

So why is that difficult? ...unless one doesn't want to accept the simple definitions, in which case they are arguing against a definition, which is pointless.

Here's a somewhat related question which I've even heard discussed on radio. I'm curious how people would explain this one to a science student in primary or secondary school.

We know that gravity acts on a refrigerator magnet. There is a downward force on the magnet, yet it does not fall. Will the magnet ever "run out" of magnetism, and if so, is this because it is expending energy to keep itself attached to the refrigerator? If it is in fact expending energy, why do we say it's not doing any "work" to stay attached?

The magnet is not expending energy. See the definitions above. Why is this hard?

After all, as my version of the question goes, if I am a first-grader...

I don't mean to be rude, but you aren't a first grader, are you? I don't think it is wise to pretend you are. This thread has gone weird and it doesn't seem to me that there is a good reason why. The definitions of these terms are short sentences. They are simple. We're not trying to define "photons" or "life" or "existence" here. What's the problem?

My muscles certainly ache after holding on to that wagon until my dad comes along and takes over. I am applying a force to the wagon, I am expending energy to keep it from moving, my energy is running low, yet physics says I am only doing "work" when I pull the wagon up the hill. Why?

Because that's the definition of "work". I don't get it: you used the definition of "force" correctly and the definition of "energy" correctly; so why don't you use the definition of "work" correctly? Is it because the energy and the work aren't equal? Well that's just efficiency.

 

[bhobba]

Actually I was thinking of a simpler explanation!

You can explain it all to a 10 year old - but it's trickier - that's all eg what do you do if they ask why does the change in Kinetic energy equal the work done or why is kinetic energy 1/2mv^2 - even what is energy anyway - smart ass 10 year olds . Tell them we will return to it when you have studied calculus, hopefully by which time you are out of the picture and someone else will do it.

Thanks
Bill

 

[Aufbauwerk 2045]

I don't see why. Work is force times distance, which is the mechanical expending of energy. And force is just what a scale reads.

So why is that difficult? ...unless one doesn't want to accept the simple definitions, in which case they are arguing against a definition, which is pointless.

The magnet is not expending energy. See the definitions above. Why is this hard?

I don't mean to be rude, but you aren't a first grader, are you? I don't think it is wise to pretend you are. This thread has gone weird and it doesn't seem to me that there is a good reason why. The definitions of these terms are short sentences. They are simple. We're not trying to define "photons" or "life" or "existence" here. What's the problem?

Because that's the definition of "work". I don't get it: you used the definition of "force" correctly and the definition of "energy" correctly; so why don't you use the definition of "work" correctly? Is it because the energy and the work aren't equal? Well that's just efficiency.

I think it's obvious that I'm asking from a pedagogical standpoint. I heard a brilliant physicist try to explain the fridge magnet issue to a radio talk show caller, and he had a terrible time putting it into plain English. I am curious if other people can explain it, even to the proverbial ten year old. Are some people not familiar with that concept? Actually I've heard other versions, such as it was Einstein and it was a six year old. But I think most people really do understand the question. Sorry, if it's not clear to people by now, I can't clarify it any more. Thanks.

 

[russ_watters]

I think it's obvious that I'm asking from a pedagogical standpoint..

It wasn't obvious to me and I suspect was not obvious to others who responded to you. The devils' advocate act is not productive if we already have a devil. It obfuscates the lesson we're trying to teach.

I heard a brilliant physicist try to explain the fridge on the magnet issue to a radio talk show caller, and he had a terrible time putting it into plain English.

That's shocking. The issue is breathtakingly simple.

I am curious if other people can explain it, even to the proverbial ten year old. Are some people not familiar with that concept? Actually I've heard other versions, such as it was Einstein and it was a six year old. But I think most people really do understand the question. Sorry, if it's not clear to people by now, I can't clarify it any more. Thanks.

Is this more devil's advocate or are you actually not clear on the issue? I really can't tell. If I explain it in a way a 10 year old can understand, will you come back and say that's not good enough because you're only six?

 

[mfig]

We start from displacement, time, velocity and then force, but I don't get why would we define a physical quantity called 'work' as force times distance?

Work connects the concepts of net force and energy, as was eluded to earlier. Recall:

$W_{a,b} = \int_a^b \textbf{F}\cdot d \textbf{x} = \int_a^b m \frac{d \textbf{v}}{dt}\cdot d \textbf{x} = \int_a^b m d \textbf{v} \cdot \frac{d \textbf{x}}{dt} = \int_a^b m d \textbf{v} \cdot \textbf{v} = \int_a^b \frac{m}{2} d (\textbf{v} \cdot \textbf{v}) = \int_a^b \frac{m}{2} d (v^2) = \int_a^b d( \frac{m v^2}{2}) = \Delta KE_{a,b}$

This connection is vital to a fuller view of the physics. That is good enough reason to define work the way it is, but I think there are other reasons too. It does comport with our experience that when we push something, i.e., we exert a force on an object through a distance, we get tired and feel like we have done work. Think about throwing a baseball from 0 to 90 mph over a 2.5 meter distance, as a pitcher does. It takes work to do that.

 

[PeterDonis]

It does comport with our experience that when we push something, i.e., we exert a force on an object through a distance, we get tired and feel like we have done work.

But, as @Aufbauwerk 2045 pointed out, we also get tired and feel like we have done work if we haven't actually moved anything through any distance--i.e., in a case where, to a physicist, we haven't done any work. So it's important to keep the physicist's concept of work separate from the ordinary common language concept of work. They're not the same.

 

[mfig]

But, as @Aufbauwerk 2045 pointed out, we also get tired and feel like we have done work if we haven't actually moved anything through any distance--i.e., in a case where, to a physicist, we haven't done any work. So it's important to keep the physicist's concept of work separate from the ordinary common language concept of work. They're not the same.

I agree that they are not the same, that is why I said the definition comports with our experience. I didn't mean to imply that it exactly matches our experience. Also, I am not sure I agree that to a physicist there is no work done when you strain against an immovable object. Even though your muscle fibers are not moving the wall, say, they are moving as they contract against your bones while you exert yourself. Thus I think a case can be made that is work being done by your muscles, it just doesn't translate through the usual channels to impact an external object.

 

[Aufbauwerk 2045]

I looked up what Feynman had to say about this.

http://www.feynmanlectures.caltech.edu/I_14.html

More references.

https://physics.stackexchange.com/questions/277347/what-is-behind-the-definitions-of-work-and-energy

 

[phinds]

I looked up what Feynman had to say about this.

http://www.feynmanlectures.caltech.edu/I_14.html

Where he says exactly what has been said repeatedly in this thread:

The word “work” in physics has a meaning so different from that of the word as it is used in ordinary circumstances that it must be observed carefully that there are some peculiar circumstances in which it appears not to be the same. For example, according to the physical definition of work, if one holds a hundred-pound weight off the ground for a while, he is doing no work. Nevertheless, everyone knows that he begins to sweat, shake, and breathe harder, as if he were running up a flight of stairs. Yet running upstairs is considered as doing work (in running downstairs, one gets work out of the world, according to physics), but in simply holding an object in a fixed position, no work is done. Clearly, the physical definition of work differs from the physiological definition, for reasons we shall briefly explore.

 

[CWatters]

Of course force, energy, and work can be confusing concepts. Here's a somewhat related question which I've even heard discussed on radio. I'm curious how people would explain this one to a science student in primary or secondary school.

We know that gravity acts on a refrigerator magnet. There is a downward force on the magnet, yet it does not fall. Will the magnet ever "run out" of magnetism, and if so, is this because it is expending energy to keep itself attached to the refrigerator? If it is in fact expending energy, why do we say it's not doing any "work" to stay attached?

The magnet is held up by friction. The magnet supplies the normal force needed for there to be friction force to counter gravity. It is not necessary to expend energy to create a force. For example a book exerts a force on a book shelf without expending any energy. Likewise the fridge magnet expends no energy creating a force between it and the fridge.

After all, as my version of the question goes, if I am a first-grader standing near the top of a hill, pulling on my red wagon, where my little sister happens to be sitting, just enough to keep it from moving so it won't run downhill, because I'm too weak to pull it any farther uphill, then I am doing "work" as we normally understand it. My muscles certainly ache after holding on to that wagon until my dad comes along and takes over. I am applying a force to the wagon, I am expending energy to keep it from moving, my energy is running low, yet physics says I am only doing "work" when I pull the wagon up the hill. Why?

If you put a rock under the wheels or tie the waggon to a rock it wouldn't roll down the hill either right? The rock wouldn't consume any energy to do this so there must be something different about you and a rock. The answer is that a human being is a very inefficient machine. It burns energy when creating forces even when those forces do no useful work.

 

[bhobba]

I don't get it: you used the definition of "force" correctly and the definition of "energy" correctly; so why don't you use the definition of "work" correctly? Is it because the energy and the work aren't equal? Well that's just efficiency.

All you wrote is true. But the following are legit issues depending on how on the ball the person you are explaining it to is. They are:
1. What is energy?
2. Why is 1/2 mv^2 a form of energy?
3. Why is the work energy theorem true? That one just requires a bit of algebra (plus using a few very early calculus concepts such as Δx so small you can neglect (Δx)^2 to prove the first equation):
https://courses.lumenlearning.com/boundless-physics/chapter/work-energy-theorem/

The other two are trickier - requiring calculus and Noether which would not be understandable to a 10 yo (unless you are Terry Tao).

For 1 and 2 you just have to assure them there is a general definition of energy but it requires more advanced math (tell them it's calculus - it may motivate them to study it later). Show them the interesting story of Noether and explain the theory in words. Tell them for a free particle applying Noether leads to energy of 1/2mv^2 but to prove it will need to be deferred until later.

Its interesting there is a book accessible to grade 12's that does that - it was designed for that exact purpose - first year Harvard students with Calculus BC could be exposed to what is considered advanced classical mechanics from the start.

Thanks
Bill

 

[fresh_42]

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[PeterDonis]

I am not sure I agree that to a physicist there is no work done when you strain against an immovable object.

There is no work done on the object, which was the point under discussion. It is true that your muscles are doing work if you include the internal motions of parts of your body in the model.

 

[Monsterboy]

After all, as my version of the question goes, if I am a first-grader standing near the top of a hill, pulling on my red wagon, where my little sister happens to be sitting, just enough to keep it from moving so it won't run downhill, because I'm too weak to pull it any farther uphill, then I am doing "work" as we normally understand it. My muscles certainly ache after holding on to that wagon until my dad comes along and takes over. I am applying a force to the wagon, I am expending energy to keep it from moving, my energy is running low, yet physics says I am only doing "work" when I pull the wagon up the hill. Why?

This has sparked of an interesting conversation although some might find it silly. It is difficult to explain this to a six year old without proper context.
Let me give an alternative solution. You are doing work by pulling the wagon but you are not doing work on the wagon, work is being done inside your own body by expending the internal energy present in your body to breathe air using your lungs and pump blood through your blood vessels to transport oxygen and glucose to your muscles. Now, that is considered as work because you are pumping the fluid throughout your body i.e flow work.

It is just like holding down the accelerator and applying the breaks fully at the same time, you are not any work but you are wasting the fuel i.e energy. But inside the car the crank shaft keeps rotating and that is rotational work being done inside the car body without the car as a whole doing any work.

You might argue that you are doing flow work in your body even while sitting still in your room, that is also correct, we are constantly consuming our internal energy in our body to run our internal organs and the body as a whole, that requires our blood to do flow work, but you will be doing less work in this case because your muscles aren't consuming as much energy.

 

[FactChecker]

Whatever the definition is, it must not say that a table holding up a book for a month is doing "work" all that time. If a spring is squeezed between two fixed surfaces, we must not say that both the spring and the fixed surface are doing (opposing??) "work". With a definition like that, "work" would be a useless concept.

 

[PeterDonis]

You are doing work by pulling the wagon but you are not doing work on the wagon, work is being done inside your own body

This is fine if your model includes all the internal motions of your own body, presumably because you are interested in them for some reason. But if all you're interested in is the motion (or lack thereof) of the wagon, talking about "work being done inside your own body" just adds unnecessary detail. That's why physicists will typically prefer to just say that if the wagon is not moving, no work is being done, and leave it at that.

 

[Monsterboy]

This is fine if your model includes all the internal motions of your own body, presumably because you are interested in them for some reason. But if all you're interested in is the motion (or lack thereof) of the wagon, talking about "work being done inside your own body" just adds unnecessary detail. That's why physicists will typically prefer to just say that if the wagon is not moving, no work is being done, and leave it at that.

Agreed, I was just addressing the question of "If I am not doing any work, why am I feeling tired ?" in the context of an overthinking 1st grader who asks too many questions and my answer is "you are feeling tired because you spending your internal energy to do internal work to apply a force on the wagon, but the force you are applying on the wagon is balanced by the force of gravity."

 

[Grinkle]

"you are feeling tired because you spending your internal energy to do internal work to apply a force on the wagon, but the force you are applying on the wagon is balanced by the force of gravity."

The first grader promptly kicks you in the shin. ;-)

 

[Aufbauwerk 2045]

Yes, It is often the case that science uses common words in different or more restrictive or differently defined ways than regular English does. Get used to it.

If it is "often the case," can you provide many other examples from classical physics at least? Modern physics is another story. But classical physics deals with our familiar macroscopic world.

I think the attitude of "get used to it" may work in some subjects, but to me it seems to be dismissing the OP's question in a way that is not appropriate for a science forum.

 

[russ_watters]

If it is "often the case," can you provide many other examples from classical physics at least?

Speed, velocity, weight, mass, elasticity. Basically all of the concepts in classical physics that are used colloquially are misused or used imprecisely.

Heck, in Pennsylvania, the answer to the question: "How far is it to Wawa?" is "5 minutes!"

I think the attitude of "get used to it" may work in some subjects, but to me it seems to be dismissing the OP's question in a way that is not appropriate for a science forum.

It's not [inappropriate]. "Get used to it" is the first barrier to entry to any serious subject. It means: this subject has a specific vocabulary that you must understand and use properly in order to even communicate with people about it.

 

[Aufbauwerk 2045]

There is no work done on the object, which was the point under discussion. It is true that your muscles are doing work if you include the internal motions of parts of your body in the model.

The point under discussion is the OP's question.

He begins as follows.

"So, here we are trying our best to describe physical universe. We start from displacement, time, velocity and then force, but I don't get why would we define a physical quantity called 'work' as force times distance?"

This is a legitimate question. It is clear, from at least one previous thread on this forum, as well as from numerous discussions on this point on the internet, that it's not a rare question.

"Is it just some quantity we defined in physics because it turns out to be useful and it doesn't have much of reference with idea of 'work' in usual meaning itself?"

This is a two-part question. Do we define 'work' the way we do because it is useful? The answer is 'yes.' Does this usage not have much reference to the idea of 'work' in its usual meaning? It certainly has some reference, but our common understanding of the word does not match the way physics uses it.

This raises the question why do we use this word "work," which is so familiar to us, in a way that goes against our common idea of "work." It is obvious from the many discussions one finds on this issue, including an old thread on this forum, that this use of the word "work" in physics is problematic. It may not be the best terminology. This is not to say we can change it at this point. A curious person would want to know when was the word first used the way we use it. This is a history of science question. Some people may be curious about the history of one of the most basic terms we use in physics.

It is not a good answer to simply dismiss the question by saying that's just the way it is, so "get over it." That is not an attitude that respects someone's desire to understand why a certain word is used. "Get over it" is not the attitude that promotes curiosity and digging more deeply into why things are the way they are.

 

[Aufbauwerk 2045]

Of course the practical engineer and the compulsive questioners known as scientists may approach this whole subject in different ways. If I belonged to an engineering forum, I would not even bring the subject up. I would not say "get used to it" because it seems a bit rude. But I would not deviate from the program. I would just refer to the equations and say the important thing from an engineering standpoint is to be able to use the equations in the real world. I've studied and worked on problems from both viewpoints, so I do understand the difference. This just happens to be a physics forum.

 

[FactChecker]

If the OP thinks hard about the common definition that he is used to, he will see a huge number of problems. He would have to distinguish between a person holding a weight overhead (difficult), versus holding it in a horizontally extended arm (very difficult), versus holding it with his arm down (easy). He would also have to distinguish between a weight being held up by a person (doing work) versus by a desk (no work). There are endless problems with that definition. I think it is untenable.
And the OP would have to develop that theory enough to compete with the standard physics definition, which is intimately related to energy and is useful throughout physics.
As has been said, this question has been discussed before many times. We have accepted the use of the word "work" as it is currently defined in physics.

 

[PeterDonis]

It is obvious from the many discussions one finds on this issue, including an old thread on this forum, that this use of the word "work" in physics is problematic.

No, what is obvious is that, as you say, the physicist's usage of the word "work" is different from the ordinary language usage of the word "work". But that's true of pretty much every substantive term used in physics. And it's that way for a very good reason. Physicists don't use words with technical meanings different from their ordinary ones just to be difficult. They do it because the ordinary language meanings of words are not suitable for describing physics. A key part of learning physics is unlearning intuitions, and many of those intuitions come from ordinary language and the meanings it gives to words.

It may not be the best terminology.

But here's the problem: there is no best terminology in the sense you are using the term. There is no way to either pick words whose ordinary language meanings will work for describing physics, or find words that are rare enough in ordinary usage that giving them technical meanings in physics won't impact the usage in describing physics of any common ordinary words. That is part of what I was referring above when I said that the ordinary language meanings of words are not suitable for describing physics.

It is not a good answer to simply dismiss the question by saying that's just the way it is, so "get over it."

That's not what people are saying. What people are saying is what I said above.

 

[Aufbauwerk 2045]

A question for the elementary school and middle school science teachers out there. Are the topics of work and conservation of energy taught in your school? Is it part of Common Core? If they are taught, how are they taught? What do you think about the Feynman essay I linked to about math and science textbooks? I like his point about not using certain standard physics terms to explain things to kids, because in fact it does not really explain anything.

 

[PeterDonis]

What do you think about the Feynman essay I linked to about math and science textbooks?

Since that link was actually in a post that got deleted, I am re-posting it here:

https://fs.blog/2016/07/richard-feynman-teaching-math-kids/

 

[Aufbauwerk 2045]

I found at least part of the answer I was looking for. I asked a few posts ago whether our definition of work originated with the study of machines. People a few centuries ago obviously did not come up with the definition based on Noether. They were looking for something useful in their own time based on the physics they knew.

It seems that Coriolis is responsible. "Calculation of the Effect of Machines, or Considerations on the Use of Engines and their Evaluation. (1829)"

Since I'm so interested in how ideas develop, I'll add that to my reading list. The engineers are free to ignore it and get on with their work!

:)

 

[PeterDonis]

I like his point about not using certain standard physics terms to explain things to kids, because in fact it does not really explain anything.

That's not the point he was making. The point he was making was that if you use a term, whether it's a "standard physics term" or not, you should link the term to something the child can observe and understand. In the example of the toy, saying "energy makes it move" tells the child nothing because he can't observe energy making it move and can't understand how "energy makes it move" links to anything he can observe. That's not a problem with the word "energy"; it's a problem with the general method of teaching that Feynman is describing, and would be the same even if technical physics terms were carefully avoided.

To illustrate a different way, suppose that the examples of the wind-up toy dog, the real dog, and the motorcycle were presented as follows:

"If we look inside the wind-up toy dog, we see that winding it up coils up a spring, and then when we release the dog the spring uncoils and makes the toy dog move."

"We can't take the real dog apart, but if we could, we would see that there are muscles inside the dog that are something like the spring in the toy dog: they pull on the dog's bones and move his legs and make him move. And for the dog's muscles, and all the other things inside him like his heart and his lungs and his brain, to work, the dog has to eat and has to breathe; in other words, food and air has to be put into the dog from the outside. That's something like the winding up of the toy dog."

"If we take the motorcycle engine apart, we see pistons inside that get pushed by tiny explosions inside the cylinders, and turn a shaft that turns a chain that turns the wheels and make the motorcycle move, just like the coiled spring uncoils and makes the toy dog move or the real dog's muscles pull on his bones and make him move. And for all that to work, the motorcycle has to have gas put into its gas tank from outside. That's something like the real dog eating and breathing and the toy dog being wound up."

"So in all three of these cases, we see that, in order for the thing to move, something from outside has to be put in and stored inside the thing: the spring coiled up in the toy dog, the food and air inside the real dog, and the gas in the motorcycle's gas tank. The actual things that are stored look very different, but they all have something in common: making things move. Scientists have a word for this common property of making things move: energy. So the scientists would say that the coiled up spring, the food and air inside the dog, and the gas in the motorcycle's gas tank, all contain energy."

Here I've still used the physics word--energy--but presented at the end, as a name for something already illustrated by the examples.

 

[russ_watters]

T
This raises the question why do we use this word "work," which is so familiar to us, in a way that goes against our common idea of "work." It is obvious from the many discussions one finds on this issue, including an old thread on this forum, that this use of the word "work" in physics is problematic. It may not be the best terminology. This is not to say we can change it at this point. A curious person would want to know when was the word first used the way we use it. This is a history of science question. Some people may be curious about the history of one of the most basic terms we use in physics.

It is not a good answer to simply dismiss the question by saying that's just the way it is, so "get over it." That is not an attitude that respects someone's desire to understand why a certain word is used. "Get over it" is not the attitude that promotes curiosity and digging more deeply into why things are the way they are.

This is just so wrong. It's like going to a baseball game and instead of learning how the game works, starting an argument with the umpire about his usage of the word "ball". The audacity one has to have to tell him he's using it wrong!

 

[russ_watters]

But here's the problem: there is no best terminology in the sense you are using the term. There is no way to either pick words whose ordinary language meanings will work for describing physics, or find words that are rare enough in ordinary usage that giving them technical meanings in physics won't impact the usage in describing physics of any common ordinary words.

Well, as @jbriggs444 said: We could call it "grobnitz". That's only half a joke. It means that words are just a jumble of letters that we assign a meaning to, by convention. So why didn't/don't physicists invent new words instead of co-opting existing words? Because there's no harm in using a word that's almost used the way they need it and modifying it just slightly to make it what they need it to be. And it's so much easier to remember an already existing word.

This is a feature, not a bug!

 

[PeterDonis]

This is a feature, not a bug!

I agree. I was just pointing out that, whether you take this obvious route (of co-opting existing words for all the good reasons you give) or invent new words like "grobnitz", you're never going to get to the point @Aufbauwerk 2045 seems to want to get to, where you can somehow just use ordinary language and still have an accurate description of the physics.