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

[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 defintion 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 occured, 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.

 

[Buckleymanor]

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 occured, 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.

Well I am trying to imagine an idealized suface which has 0 ductabilty or yeild.
Only problem is it don't exist .
Imagine if it did, we would have 0 ductable rods being pushed all around the universe at greater than lightspeed.
Anyway if we want to work with the physics of the problem I suguest we don't include something that don't exist and has nothing to do with this thread or anything.

 

[Frame Dragger]

Well I am trying to imagine an idealized suface which has 0 ductabilty or yeild.
Only problem is it don't exist .
Imagine if it did, we would have 0 ductable rods being pushed all around the universe at greater than lightspeed.
Anyway if we want to work with the physics of the problem I suguest we don't include something that don't exist and has nothing to do with this thread or anything.

Idealized and Toy models are nothing new in physics or science. We agree and realize that such doesn't exist, and use the elements of the model that apply. In this case, my point was to focus you on the issue at hand. Obviously I'm not up to the task, but I'm sure Doc Al will clarify his earlier statement to your satisfaction.

 

[Buckleymanor]

Idealized and Toy models are nothing new in physics or science. We agree and realize that such doesn't exist, and use the elements of the model that apply. In this case, my point was to focus you on the issue at hand. Obviously I'm not up to the task, but I'm sure Doc Al will clarify his earlier statement to your satisfaction.

Apreciated I realize that my reply was a bit picky but the OP is idealised in trying to insist that no work is done.
You might as well say there are two tug of war teams and a rope, who have been pulling against each other for an hour, why is it tiring for them to pull even though no work is done.

 

[Doc Al]

Why would the wall not be moveing.

Because it has reached the point of maximum compression.

Once you stop pushing or relax a bit the wall would push back with the same force.

The wall always pushes back with the same force that you push on it.

To maintain compression you would have to keep pushing.

So? That's still not doing work. Work is force through a displacement. Once the displacement stops--when you've reached maximum compression--you no longer do any work.

Apreciated I realize that my reply was a bit picky but the OP is idealised in trying to insist that no work is done.

And that's pretty much true. Except for the trivial bit of work done on the wall when it first compresses after you start to push, no work is done.

You might as well say there are two tug of war teams and a rope, who have been pulling against each other for an hour, why is it tiring for them to pull even though no work is done.

Yes! It's the exact same problem. The reason that it's tiring has nothing to do with the mechanical work they are doing on the rope--since they aren't doing any work unless the rope moves.

 

[Frame Dragger]

I think the issue here is that Buckleymanor is not talking about mass/distance Work, but rather the lay view of work. Kind of unforgivable given that this is page 4 of the thread, but that's my impression at this point.

 

[Buckleymanor]

So? That's still not doing work. Work is force through a displacement. Once the displacement stops--when you've reached maximum compression--you no longer do any work.

I think I am confused, because you are upright and pushing how does the displacement stop.
Ok. so if you mounted a spring scale a small amount of work would be done via gravity.
Once the displacement stopped on the scale as it took your weight and maximum compression was reached you would no longer do any work.
However if you were stood upright and pushing against a wall or spring scale, how would you reach maximum compression you would allways be able to push a little harder.
Thanks.

 

[Doc Al]

I think I am confused, because you are upright and pushing how does the displacement stop.

When you push a wall, you think it keeps moving? How far does it travel?

However if you were stood upright and pushing against a wall or spring scale, how would you reach maximum compression you would allways be able to push a little harder.

Maximum compression for the force you are exerting. Sure, if you push harder, it will compress a bit more.

Just to be clear: Say you are going to press against the wall with a force of 100 N. As you start to press the wall (force just above zero) the wall begins to microscopically compress. Once you've gotten to your maximum force (just 100 N in this case), the wall is compressed by some tiny distance. As long as you keep pushing with the same force, the wall is no longer moving.

 

[Andy Resnick]

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.

Be careful here- if you are talking about a (nearly) rigid wall, like a brick or cinderblock wall, I don't think it makes sense to talk about elastic compression, especially when compared to your hands. If the wall can elastically deform (perhaps a plywood wall), then yes- you have done work deforming the wall, and that energy will be returned to you when you stop pushing. If the wall plastically deforms (styrofoam walls?), then you do work on the wall which is then dissipated due to the plastic deformation. Microscopically, we do not yet understand how that process occurs- crack formation, specifically.

 

[Andy Resnick]

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.

Do you include the possibility of an isothermal increase in entropy?

 

[Buckleymanor]

When you push a wall, you think it keeps moving? How far does it travel?



Maximum compression for the force you are exerting. Sure, if you push harder, it will compress a bit more.

Just to be clear: Say you are going to press against the wall with a force of 100 N. As you start to press the wall (force just above zero) the wall begins to microscopically compress. Once you've gotten to your maximum force (just 100 N in this case), the wall is compressed by some tiny distance. As long as you keep pushing with the same force, the wall is no longer moving.

I don't think the wall travels as such only in as much that it is compressed microscopically.

The OP was not clear about how many Newtons he just says push hard.
It also don't mention to keep pushing with the same force.

 

[Doc Al]

I don't think the wall travels as such only in as much that it is compressed microscopically.

The OP was not clear about how many Newtons he just says push hard.
It also don't mention to keep pushing with the same force.

Regardless, for typical pushes against presumably rigid walls, the displacement of the point of contact will be tiny and will stop once you reach the maximum force you are willing to push. (We're not talking about pushing with infinite force!) The mechanical work done on the wall during compression will be tiny--and finite--and has little to do with the topic of this thread.

 

[Buckleymanor]

Regardless, for typical pushes against presumably rigid walls, the displacement of the point of contact will be tiny and will stop once you reach the maximum force you are willing to push. (We're not talking about pushing with infinite force!) The mechanical work done on the wall during compression will be tiny--and finite--and has little to do with the topic of this thread.

Records will allways be broken might as well cancel the next set of games.
Thanks for your answers.

 

[Doc Al]

Records will allways be broken might as well cancel the next set of games.

What does breaking records have to do with understanding the physics in this thread?

 

[Buckleymanor]

What does breaking records have to do with understanding the physics in this thread?

Not much point in explaining as the parameters of this thread have allready be set.
As you have allready pointed out it will have little to do with the topic.

 

[BL4CKCR4Y0NS]

There is a workout course called 'isometrics'. This is basically what you have just asked.

Isometrics is basically doing an action or putting yourself into a position where you will not move, but your muscles are working. Over time, the muscles WILL GET TIRED EVEN IF YOU ARE NOT MOVING which is said in:

your body doesn't need to be doing any work to use energy

Or maybe a better way to say it is ... your body does not need to be MOVING to use energy

 

[Sprinkle159]

Not sure if this helps you, but it it helped me a little. (I'm right where you are in the work and kinetic energy chapter)

"...and because the objects do not behave as particles, it is generally not correct to apply the particle form of the work-energy theorem to objects subject to frictional forces."
(The example is 2 cars crashing. Resnick, Halliday, Krane Volume 1 Physics)

If your body and the wall aren't particles, and the wall your pushing on is also experiencing a large frictional force from the ground it's attached too, then i think this is saying you can't apply the work energy theorem as its been given to us.

It goes on to say we will find out how to deal with complex systems like cars and walls in a few chapters so you might also try looking there.
(take everything I've said here with a very large grain of salt please)