Why no work when carrying piano on your back down a hallway?

[Karagoz]

In a physics articles and texts they say

You don't perform a work when carrying a heavy object over a long corridor
or
if you are carrying a piano on your back down a hallway, you are not actually doing any real work

But why?

Since you're using force and energy to move an object with mass, aren't you doing some work?

 

[phinds]

In a physics articles and texts they say

You don't perform a work when carrying a heavy object over a long corridor
or
if you are carrying a piano on your back down a hallway, you are not actually doing any real work

But why?

Since you're using force and energy to move an object with mass, aren't you doing some work?

What is the definition of work?

 

[Karagoz]

What is the definition of work?

Work:

W = Fs

 

[phinds]

Work:

W = Fs

simply stating a formula show no understanding unless you at least define the terms and show how they apply in the situation you are analyzing.

 

[Karagoz]

simply stating a formula show no understanding unless you at least define the terms and show how they apply in the situation you are analyzing.

Work:

Work = Force * displacement.

When the boy carries a bag, and moves the bag 10 meters forward horizontally, he uses force to the bag to move the bag.

That force * 10 meters is the work.

The gravity is not doing any work to the bag, but the guy is doing work to the bag by carrying and moving it.

 

[russ_watters]

When the boy carries a bag, and moves the bag 10 meters forward horizontally, he uses force to the bag to move the bag.

Can you explain more about that force? Where does it come from, what direction is it applied in, what reaction force opposes it?

 

[Karagoz]

When you put mass on wheel and move it horizontally, aren't there any forces the mass?

Like in this pic.

 

[russ_watters]

When you put mass on wheel and move it horizontally, aren't there any forces the mass?

Like in this pic.View attachment 211698

Depends: is there friction there, such as in the wheels? If there is no friction or other reaction force, what does your applied force do?

 

[Karagoz]

There are frictions in the wheel and air friction.

When we walk, we use force to move forward. And we face some friction that slows us down.

When we carry something and walk, won't we then apply forces to what we carry?

 

[jbriggs444]

Work = Force * displacement.

Work = Force * displacement times the cosine of the angle between the two.

If you lift an object straight up, the movement is vertical and the force is vertical and equal to the weight of the object.

If you pull a wagon along the sidewalk, the movement is horizontal and the force is horizontal but is not equal to the weight of the object. If the axles are well greased, you do very little work pulling it.

The support force of the wagon wheels is vertical and is equal to the weight of the wagon plus load. But the displacement is at right angles to the force, so no work is done by the wagon wheels.

If you walk a piano down the sidewalk on your back, your muscles will be straining, alternately contracting and expanding. Muscles expend energy during contraction but do not reclaim it when expanding. You expend/waste energy while doing negligible "work".

 

[russ_watters]

There are frictions in the wheel and air friction.

How much force do you think those generate? We can make reeally good wheels and bearings...

...and when carrying a piano, you don't even have wheel friction.

When we walk, we use force to move forward. And we face some friction that slows us down.

How/how much?

When we carry something and walk, won't we then apply forces to what we carry?

We're trying to make you specifically define and better yet quantify the actual forces. I think if you try you will have a hard time defining any relevant forces that can't be arbitrarily close to zero.

 

[Karagoz]

"If you hold a book with your palms facing up and the book on top while walking forward, are you doing work on the book?"

"since the friction between your hands and the book is in the direction of motion, yes you are doing work on the book"

Quoted from DrewD from this link: https://www.physicsforums.com/threa...book-while-i-am-holding-it-and-moving.685772/

But if that's the case, then what's the reaction force to the book?

 

[russ_watters]

"If you hold a book with your palms facing up and the book on top while walking forward, are you doing work on the book?"

"since the friction between your hands and the book is in the direction of motion, yes you are doing work on the book"

Quoted from DrewD from this link: https://www.physicsforums.com/threa...book-while-i-am-holding-it-and-moving.685772/

But if that's the case, then what's the reaction force to the book?

Did you read the responses?

Physicists like being pedantic, but I think it confuses the issue. I'm an engineer: Almost zero is basically zero, so to me the correct answer is just zero.

 

[phinds]

@Karagoz you really seem to be doing your best to avoid the fundamental issue of the question you asked. Had you asked "why is it that when you are carrying a piano on your back down a hallway, you are doing only a really trivial amount of work, if any at all", then all this quibbling might be justified, but that's not really the heart of what you asked.

 

[Karagoz]

Did you read the responses?

Physicists like being pedantic, but I think it confuses the issue. I'm an engineer: Almost zero is basically zero, so to me the correct answer is just zero.

I read the responses. None did refused his points.

 

[russ_watters]

I read the responses. None did refused his points.

Right, but to me the important question is, "how much?" If all you have learned so far is "not zero", then I suspect that isn't really what you wanted to know and likely even contains hidden errors.

 

[Karagoz]

Assume there's a box weighing 100 KGs on the ground, and there's absolutely no friction in the ground and it's just horizontal ground. And there's no air drag either.

If you put the box on that ground, then kick the box, the box will start moving. Some work is needed to get the box start moving.

But since no force is needed to keep it moving forward (only force from the kick to get it start moving), even if the box did slide 100 KMs forward (because there's no friction and air drag), the work done will be just zero?

 

[jbriggs444]

Assume there's a box weighing 100 KGs on the ground, and there's absolutely no friction in the ground and it's just horizontal ground. And there's no air drag either.

If you put the box on that ground, then kick the box, the box will start moving. Some work is needed to get the box start moving.

But since no force is needed to keep it moving forward (only force from the kick to get it start moving), even if the box did slide 100 KMs forward (because there's no friction and air drag), the work done will be just zero?

No work during the journey. There was work done by the initial kick. That work will be equal to the kinetic energy of the box while it is sliding.

 

[Karagoz]

No work during the journey. There was work done by the initial kick. That work will be equal to the kinetic energy of the box while it is sliding.

So in that imaginary case as I described, even if the box did slide 100KMs or 5KMs, the "work" will be the same (which will only be the initial kick, and we assume the force of the kick is same).

And a person carrying a box on his hand (like in this picture):

Only initially (which is the friction between his hand and the box) is some force and work needed, and after that no more horizontal force is needed keep the box moving forward (if we assume no air drag)?

 

[jbriggs444]

And a person carrying a box on his hand (like in this picture):
[...]
Only initially (which is the friction between his hand and the box) is some force and work needed, and after that no more horizontal force is needed keep the box moving forward (if we assume no air drag)?

Yes.

Edit: as @russ_waters points out, physicists like being pedantic. The equation on the picture is incorrect. While it is correct that $W = \vec{F} \cdot \vec{x}$, it is not true that this is equal to $0 \cdot \vec{x}$. A vector dot product yields a scalar result, not a vector.

 

[UsableThought]

In a physics articles and texts they say
You don't perform a work when carrying a heavy object over a long corridor
or
if you are carrying a piano on your back down a hallway, you are not actually doing any real work

If you walk a piano down the sidewalk on your back, your muscles will be straining, alternately contracting and expanding. Muscles expend energy during contraction but do not reclaim it when expanding. You expend/waste energy while doing negligible "work".

Just to clarify, from a pedagogical POV, shouldn't the example also clearly stipulate (rather than leave it unsaid) that the motion is idealized as being constant; e.g. as has carefully been done in presenting the examples titled "When a force does no work" on the Work page from the Hyperphysics educational site?

The risk (to the naive reader, e.g. someone like me) is that if the example includes muscles lengthening & contracting, but fails to say that biomechanical motion is being excluded, we may get hung up in the weeds of unnecessary realism. E.g. we might complain that the piano/object will be constantly accelerating & de-accelerating, and in not just one plane but three planes; this figure from a journal paper on the biomechanics of walking shows the 2D view:

Aside from this nitpick, it would seem a good idea for teachers/textbooks/classes to explore early on the pivotal shift in the history of science, starting with physics, toward simplifying or abstracting situations to analyze them & create models; and perhaps Aristotle vs. Galileo/Newton will be mentioned w/ regard to motion. If this is done it ought to help students spot idealized examples more readily.

I bring all this up mostly because the bit about the piano got to me. I have seen many pianos moved over the years, including someone getting under a baby grand and humping it along (not "walking") on their back a very short ways across a living room; and it's always a stop-start, stop-start, up-down, up-down affair. Just this past Friday I watched three strong men at our local concert hall disassemble and move a rented 900-lb concert grand back into the truck to go to Yamaha in NYC. It was protracted labor with lots of lengthy pauses for regrouping & discussing what motion to attempt next. Constant motion, not.