Why is it tiring to push hard against a solid wall even though no work is done?

[Frame Dragger]

I just want to make sure I understand what you are saying: If I remove an engine from a car, put it on a test stand, and operate that engine, it now has an efficiency of precisely zero?

Uhhhh... Ok... if that's the definition we're working with, my posts no longer make sense.

 

[DaveC426913]

I just want to make sure I understand what you are saying: If I remove an engine from a car, put it on a test stand, and operate that engine, it now has an efficiency of precisely zero?

If the engine's operation does not result in a mass displaced by a distance then work done is zero.

 

[Buckleymanor]

Why is it tiring to push hard against a solid wall even though no work is done?

Because the wall pushes back with the same amount of force or else it would collapse.

 

[Frame Dragger]

If the engine's operation does not result in a mass displaced by a distance then work done is zero.

The mass is displaced... pistons move, belts spin. The only difference is that work isn't going into a transmission so that forward motion of the engine block occurs. Like the human body, an idling engine has many subunits doing work and Work. Maybe you could say the block as a whole isn't doing work if it isn't moving... then again, if you turn on this fictional engine and don't bolt it down it's going to move all over the place.

 

[rewebster]

if you push on the wall and nothing happens, it just means that you're not strong enough to break the molecular bonds that hold the wall in that position/structure.

If you were as strong as, say a D8 Cat bulldozer, then the wall may move.

If it was a rock wall with a lot of other rock behind it, it may take more than even a D8.

 

[Andy Resnick]

If the engine's operation does not result in a mass displaced by a distance then work done is zero.

Then by your reckoning, every single turbine and generator in the world does zero work. Clearly, that's silly.

Work is a *thermodynamic* concept, not a *mechanical* concept.

 

[Mad_Eye]

I'm not sure: what part of "your body doesn't need to be doing any work to use energy" didn't you understand? You don't seem to be listening to what people are telling you.

I think you are not listening to what people are telling me.
It had been definitely said that there is work done here.
And beside, you comment doesn't make much sense to me since using of energy IS doing work, as much as I know.

@others,
First, I've read more than once that "energy is consumed even the work done is zero" which I definitely don't understand, as far I know work IS the transfer/using of energy.

About whether work is thermodynamics' concept, or mechanical concept, I obviously not the one to tell, but all I can say is that I've seen on Wikipedia a "Work (mechanics)" article and a "Work (thermodynamics)" article.

Another thing, someone has said that work is not frame variant, so for one thing, it's definitely is, as DaleSpam has kindly helped me understand (see my other topic).

I've got the point that there is no work done on the wall, but there is still work done inside my body, my question now is, what happens then in other frames in which there is no work done? how is the "muscles tiredness" is then explained?

I was asked about why bother dealing with other frames,
well, I think this is understanding physics, trying to challenge it, check whether it work in the weirdest situations, I think that what gives you a qualitative assurance and knowledge.

and thank you all:D

 

[Dale]

Hi Mad Eye, the first thing to do is to make sure that you understand how muscles work. Here is a decent link for that:
http://en.wikipedia.org/wiki/Sliding_filament_model

If you notice, in step 4 the actin and myosin are detached from one another. At this point, if the muscle is under enough tension it can slide back to the original position before the "power stroke" of step 3. When that happens then the work done by the power-stroke is converted entirely to heat instead of kinetic energy or external work.

 

[Andy Resnick]

I need to point out that the energy used in the power stroke from hydrolyzing ATP is not dissipated as heat. That is an essential concept to understand.

Physiological processes that hydrolyze ATP get the energy from the Gibbs free energy- specifically, the ~30 kJ/mol of energy is obtained from the excess concentration of ATP (as compared to ADP) as compared to equilibrium conditions. Hydrolysis dissipates energy not as heat, but as relaxation towards equilibrium.

http://rmp.aps.org/abstract/RMP/v69/i4/p1269_1

In addition, the sliding filament model does not explain how isometric contraction (generation of tension without a change in length) occurs.

 

[Andy Resnick]

<snip>
I've got the point that there is no work done on the wall, but there is still work done inside my body, my question now is, what happens then in other frames in which there is no work done? how is the "muscles tiredness" is then explained?
<snip>

Muscles generate work by hydrolyzing ATP; chemical energy is converted into mechanical energy (shortening the muscle). The dissipated energy is not heat but rather chemical in nature- both by driving [ADP][Pi]/[ATP] towards equilibrium, but also metabolic byproducts such as CO2. Under some conditions heat will also be generated, but to a very good approximation the human body is isothermal.

Muscle fatigue can occur in a variety of ways, from the nerve stimulus through to availability of ATP to the myosin molecules and cell damage. Much of the physiology is still not known:

http://en.wikipedia.org/wiki/Delayed_onset_muscle_soreness

 

[Mad_Eye]

my question now is, what happens then in other frames in which there is no work done? how is the "muscles tiredness" is then explained?

Hi Mad Eye, the first thing to do is to make sure that you understand how muscles work. Here is a decent link for that:
http://en.wikipedia.org/wiki/Sliding_filament_model

If you notice, in step 4 the actin and myosin are detached from one another. At this point, if the muscle is under enough tension it can slide back to the original position before the "power stroke" of step 3. When that happens then the work done by the power-stroke is converted entirely to heat instead of kinetic energy or external work.

yes, I think I understand.
so in other frames, one part might do more work, but the other part will do less work?
still seems a bit weird for me... because there still some parts that aren't doing any work, in some frames, so they are "not tired"?

what is the meaning of tiredness in energy concept anyway?

 

[Frame Dragger]

yes, I think I understand.
so in other frames, one part might do more work, but the other part will do less work?
still seems a bit weird for me... because there still some parts that aren't doing any work, in some frames, so they are "not tired"?

what is the meaning of tiredness in energy concept anyway?

I think the issue here is that we're moving from physics through biology and into psychology. The perception of "tiredness" can be caused by a buildup of 'fatigue toxins' such as lactic adid, but the body may have a lot more to give. Tiredness can also be the result of saaaaay, lengthy starvation, and a true lack of metabolic resources. Then again, you can become so depressed (or psyhotic) that you become nearly comatose.

If I were to define "tired" in terms of physics, I would say that a system is "tired" when it has reached equilibrium @ entropy; i.e. all organized energy in a system has been reduced to a state of low order, and no more work can be accomplished through thermodynamic processes.

If I were to define "tired" in terms of psychology, I would say it is the sensation, either/both mental and phsyical which acts as a means to induce rest or sleep in animals. This can be a result of healthy processess at or below abolsute metabolic limits, or it can be the result of illness or starvation. One way or another, it is the perception of a lack of ability to pursue activities which require work (including thinking).

In terms of biology... a cell which ceases to divide and initiates apoptosis, or a neuron which can't fire until it builds a charge... there are too many definitions for anyone to be useful. Generally however I would define "tired" as either "ready to die" or "needing sleep/food/etc before continuing" as an ultimate biological term. This is why it's more useful to measure metabolic "work" in terms of heat and metabolites emitted/excreted vs. intake and not distance a weight is pushed.

 

[R Power]

I can't say anything of muscles and body and I haven't read all the posts but what consfusion under the thread starter is can be solved and looked upon this way too:

You are pushing against the wall. The wall can't move because it is fixed from four sides (roof, floor, sideways). That means when you apply force friction from all these sides (especially from bottom side or floor due to heavy weight of wall ) is so great that you can't apply more force than that which friction applies on wall when you push it. So, when you do work, static friction from all sides does equal and opposite work. So you don't see any displacement but that doesn't means you do not do work.
That means wall does equal and opposite work on you. That's why you feel pushed when you try to push a wall.

 

[Doc Al]

So you don't see any displacement but that doesn't means you do not do work.

Actually, it does mean just that. No work is being done on the wall, and the wall is doing no work on you.

Of course, in order for you to push the wall plenty of things must take place within your body, including internal work as muscle fibers contract and relax to maintain tension. It's these internal things that burn biochemical energy and create the feeling of being tired.

 

[Dale]

I need to point out that the energy used in the power stroke from hydrolyzing ATP is not dissipated as heat.

If no external work is done, the kinetic energy of the body is constant, and the potential energy of the body is decreasing (reduced chemical potential energy) then by conservation of energy the balance must go into heat.

 

[Dale]

yes, I think I understand.
so in other frames, one part might do more work, but the other part will do less work?
still seems a bit weird for me... because there still some parts that aren't doing any work, in some frames, so they are "not tired"?

what is the meaning of tiredness in energy concept anyway?

Yeah, I agree with you. Tiredness is a psychological or at most a physiological concept, not a physics concept. I don't think it makes sense to partition tiredness up into different parts.

 

[Frame Dragger]

Yeah, I agree with you. Tiredness is a psychological or at most a physiological concept, not a physics concept. I don't think it makes sense to partition tiredness up into different parts.

I don't know, I think if you were to apply the word "tired" to a physical system in physics, that would just mean: no work can be done, but once was. Tired implies prior effort, so I still interpret "tired" in physics to mean entropy at equilibrium, with no potential for work. Not useful, and very misleading... as with all anthropomorphic language in physics.

 

[Andy Resnick]

If no external work is done, the kinetic energy of the body is constant, and the potential energy of the body is decreasing (reduced chemical potential energy) then by conservation of energy the balance must go into heat.

I think we are on the same page- at the risk of being pedantic- if by 'heat' you mean 'energy unavailable to do useful work', or equivalently, heat != temperature changes.

 

[Frame Dragger]

The simple way to understand this: You do only as much work as YOU exert against the wall, and the wall "matches" it. It isn't hard to push against a wall, or a car, or a ball of cotton. The "wall" confuses this issue, which is simple: human bodies are always doing work, until we die, the issue is one of degrees of effort, and that is hardly an issue for physics.

EDIT: Another way to look at this at a macroscopic level is: "Why is it tiring to push hard against a solid FORCE GAUGE...". Any scientist would look at the gauge, say, "oh, you're exerting XNewtons of force /unit-measurement, that is equivalent to Y much 'work' or lifting an apple a foot, etc." The issue here is that at first people were not seperating the two systems: chemical potential energy in the body, and total output of force that would satisfy a basic Newtonian equation of mass over distance.

 

[Buckleymanor]

Actually, it does mean just that. No work is being done on the wall, and the wall is doing no work on you.

Of course, in order for you to push the wall plenty of things must take place within your body, including internal work as muscle fibers contract and relax to maintain tension. It's these internal things that burn biochemical energy and create the feeling of being tired.

I don't see how you reach the conclusion that no work is being done on the wall and the wall is doing no work on you.
At the very least the walls structure will be compressed and so will your fingers.
I cannot understand how you can apply pressure to a molecular structure without doing work.

 

[Dale]

I think we are on the same page- at the risk of being pedantic- if by 'heat' you mean 'energy unavailable to do useful work', or equivalently, heat != temperature changes.

Sorry, I am habitually sloppy with that term although I know better. Technically heat is a transfer of energy between systems. What I meant was thermal energy.

 

[Doc Al]

I don't see how you reach the conclusion that no work is being done on the wall and the wall is doing no work on you.
At the very least the walls structure will be compressed and so will your fingers.
I cannot understand how you can apply pressure to a molecular structure without doing work.

OK, you do a little work in compressing the wall, which stops as soon as the wall stops compressing. That has little to do with the topic of this thread though.

 

[Frame Dragger]

OK, you do a little work in compressing the wall, which stops as soon as the wall stops compressing. That has little to do with the topic of this thread though.

Actually, I'm so terribly buff that I routinely overcome degeneracy pressures when I push on walls. Everything is the Pauli Exclusion Principle to me... oh yeah *flex*. Anyway, back to reality...

I think we all seem to agree on the nature of external-mechanical vs. internal-micomechanical and internal-chemical in terms of "work". From the strict definition we have the action of actin and other tissues moving mass over distance. From the sense of pressure being exerted as equivalent to the work required in some toy model, we seem to agree. From the point of view of the metabolic process, we seem to agree. Case closed?

 

[Buckleymanor]

OK, you do a little work in compressing the wall, which stops as soon as the wall stops compressing. That has little to do with the topic of this thread though.

Don't the wall stop compressing when you are too tired to push.

 

[Doc Al]

Don't the wall stop compressing when you are too tired to push.

No. When you first push on the wall it does compress a very tiny amount, thus you do a minuscule bit of work on the wall as the wall moves that tiny bit. Unless you're pushing on a flimsy wall, you'll never notice its microscopic movement. Once the wall is fully compressed, it stops moving and thus you stop doing work on it. For practical purposes you can say that the wall doesn't move and that zero work is done.

When you no longer exert the force on the wall, it will decompress, returning to its original configuration. Again, for a solid wall, you won't notice the wall moving.

 

[Buckleymanor]

No. When you first push on the wall it does compress a very tiny amount, thus you do a minuscule bit of work on the wall as the wall moves that tiny bit. Unless you're pushing on a flimsy wall, you'll never notice its microscopic movement. Once the wall is fully compressed, it stops moving and thus you stop doing work on it. For practical purposes you can say that the wall doesn't move and that zero work is done.

When you no longer exert the force on the wall, it will decompress, returning to its original configuration. Again, for a solid wall, you won't notice the wall moving.

Sorry to be a bit pedantic but is'nt the amount of compression proportional to the amount of force applied.If you were really strong the wall would move more than if you were weaker.
Allthough the amount would be microscopic it is nether the less proportional to the amount of force, the structure of the wall, and the materials used in it's construction.
Once the wall is fully compressed, it woud stop moving but you would not stop doing work on it.
For the wall would be in a state of compression and for that to be maintained you would have to keep pushing.

 

[Frame Dragger]

Sorry to be a bit pedantic but is'nt the amount of compression proportional to the amount of force applied.If you were really strong the wall would move more than if you were weaker.
Allthough the amount would be microscopic it is nether the less proportional to the amount of force, the structure of the wall, and the materials used in it's construction.
Once the wall is fully compressed, it woud stop moving but you would not stop doing work on it.
For the wall would be in a state of compression and for that to be maintained you would have to keep pushing.

No, because you'd having to be overcoming intermolecular bonds, and that requires a great deal of energy. Stop thinking about a wall... imagine an idealized surface that always matches the person pushing it, and has 0 ductility or yield. Ok?

Then we ignore this fun stuff. If you can't accpet that... then if you had a magical force gauge, it would say: Your peak deformation of the wall occurred, then it "pushed back" and yes, technically you're maintaining some microscopic deformation of the material if it's pliant, but who cares? That has NOTHING TO DO WITH THIS THREAD.

 

[Doc Al]

Sorry to be a bit pedantic but is'nt the amount of compression proportional to the amount of force applied.If you were really strong the wall would move more than if you were weaker.
Allthough the amount would be microscopic it is nether the less proportional to the amount of force, the structure of the wall, and the materials used in it's construction.

True.

Once the wall is fully compressed, it woud stop moving but you would not stop doing work on it.

Not true. Once it stops moving, work is no longer being done on it.

For the wall would be in a state of compression and for that to be maintained you would have to keep pushing.

True. But you would not be performing any mechanical work on the wall since it is not moving.

 

[Frame Dragger]

True.

Not true. Once it stops moving, work is no longer being done on it.

True. But you would not be performing any mechanical work on the wall since it is not moving.

Of course... I have to point out that muscle output is never constant, which means there would be a constant microscropic flexing back and forth as pressure exerted waxed and waned. None of that is in the spirit of the OP's question however, which is better visualized with an idealized biological piston and perfectly unyielding surface. Then we focus on the POINT being made, which is that on a practical level, the wall doesn't move, and in terms of mechanics that means Work is not being done. The fact that everyone ALWAYS attacks the thought experiment probably reinforces its value as a teaching tool.

 

[Buckleymanor]

True.

Not true. Once it stops moving, work is no longer being done on it.

True. But you would not be performing any mechanical work on the wall since it is not moving.

Why would the wall not be moveing.
Once you stop pushing or relax a bit the wall would push back with the same force.
To maintain compression you would have to keep pushing.