Is holding a box over my head work, or not?

[Rap]

When a rocket moves upwards with constant speed the engine uses force equal with the weight(air resistance excluded) hence the net force is zero but there is motion.

...but in the above rocket example after a period of time the rocket has gained height(h) and the engine of the rocket has produced work equal with mgh.Hasn't it?

The case of a rocket is tricky and we should probably avoid it. The bottom line definition of force is that it is the time rate of change of momentum (mv). Only when you assume mass is constant does F=ma. For a rocket, mass is not constant, so the total force on the rocket is F=(dm/dt)v+ma-mg. If the rocket moves at a constant velocity, then a=0 and the force is (dm/dt)v-mg and is not zero. The force is not zero, the rocket moves, therefore work is being done on the rocket. Since dm/dt is negative (the rocket is losing mass), the force is downward while the motion is upward, so the work is negative: energy is being lost by the rocket to its exhaust gases.

 

[Rap]

I appreciate the time and thought given in the responses to my original post. I think the question in it was answered most succinctly by Andy Resnick, who stated that although no mechanical work was being done, chemical work was through the consumption of chemical energy.

So the original question has been answered very satisfactorily, thank you!

Inevitably conversations tend to evolve, and so a new question (which no one has answered for me) has come up:

A. I observe, in all corners of the universe, forces being created. A star supernovas, creating forces which eject matter. A power plant burns natural gas to create forces which turn turbines. A rocket moves through space in response to exhaust forces. Cars move along roads, powered by engines. And the common theme for each of these forces is that energy was provided to create them, none of them simply sprang into being from nowhere.

B. Taking A's observation, I turn to nature and see forces like gravity.
C. I then quite naturally ask, since A's forces required energy to create, don't B's forces?

I will be perfectly content with a 'no' answer, but please provide very concrete reasons, none of this 'that's just the way it has to be' or 'it's somehow just different' evasion. Thanks!

Mass is required to create gravity and mass is energy according to the theory of relativity.

 

[Drakkith]

Mass is required to create gravity and mass is energy according to the theory of relativity.

Mass and Energy are equivalent, but they are not the same thing. And still, gravity requires no transfer of energy.

 

[Rap]

Mass and Energy are equivalent, but they are not the same thing. And still, gravity requires no transfer of energy.

As far as gravity is concerned, they are the same. If you have a gas in a container and heat it up, the kinetic energy of its molecules will increase, its gravitational mass will increase, as will its force of gravitational attraction on another massive body.

The OP says that "forces are created by energy" but this is, of course, not a good way to say it. The correct way to say it is that some stored potential energy is being converted to kinetic energy - it is being used to do work and that work involves a force. The stored energy involves a force, its just that the force is not doing any work. I think my answer was glib, and I apologize. The energy stored in a massive object is not being converted to energy when an attracted particle falls. Its own potential energy is being converted.

A star supernovas, creating forces which eject matter. A power plant burns natural gas to create forces which turn turbines. A rocket moves through space in response to exhaust forces. Cars move along roads, powered by engines. And the common theme for each of these forces is that energy was provided to create them, none of them simply sprang into being from nowhere.

The force created by a supernova is from the conversion of mass to kinetic energy. Burning gas, rocket fuel, and gasoline converts stored chemical (i.e. atomic electromagnetic) energy to kinetic energy.

 

[Drakkith]

Yes, but if you look at a case where there is a force, but no change in velocity or position, where is the energy? Like a box on the ground, gravity is still attracting it. It still exerts a force on the box, but no energy is being converted here.

 

[Rap]

Yes, but if you look at a case where there is a force, but no change in velocity or position, where is the energy? Like a box on the ground, gravity is still attracting it. It still exerts a force on the box, but no energy is being converted here.

But the force of gravity is being countered by the upward force of the ground on the box. The net force is zero, so there is no work, and so no energy is being converted.

 

[douglis]

The case of a rocket is tricky and we should probably avoid it. The bottom line definition of force is that it is the time rate of change of momentum (mv). Only when you assume mass is constant does F=ma. For a rocket, mass is not constant, so the total force on the rocket is F=(dm/dt)v+ma-mg. If the rocket moves at a constant velocity, then a=0 and the force is (dm/dt)v-mg and is not zero. The force is not zero, the rocket moves, therefore work is being done on the rocket. Since dm/dt is negative (the rocket is losing mass), the force is downward while the motion is upward, so the work is negative: energy is being lost by the rocket to its exhaust gases.

What do you mean "the rocket is losing mass"?Are you implying the loss of fuels?Well...I didn't go that far!
Just think of any case that an object moves with constant speed.Work is being done while the net force is zero.

 

[russ_watters]

All I'm saying is, why don't they require energy, like any other force I observe does. So far, you appear to have given me a disappointing 'that's just the way it is' reply.

That's nonsensical/wrong: NONE of the fundamental forces require energy to generate them. You're welcome to continue to refuse to accept this but the universe doesn't care if you do or not.

 

[thegreenlaser]

I appreciate the time and thought given in the responses to my original post. I think the question in it was answered most succinctly by Andy Resnick, who stated that although no mechanical work was being done, chemical work was through the consumption of chemical energy.

So the original question has been answered very satisfactorily, thank you!

Inevitably conversations tend to evolve, and so a new question (which no one has answered for me) has come up:

A. I observe, in all corners of the universe, forces being created. A star supernovas, creating forces which eject matter. A power plant burns natural gas to create forces which turn turbines. A rocket moves through space in response to exhaust forces. Cars move along roads, powered by engines. And the common theme for each of these forces is that energy was provided to create them, none of them simply sprang into being from nowhere.

B. Taking A's observation, I turn to nature and see forces like gravity.
C. I then quite naturally ask, since A's forces required energy to create, don't B's forces?

I will be perfectly content with a 'no' answer, but please provide very concrete reasons, none of this 'that's just the way it has to be' or 'it's somehow just different' evasion. Thanks!

If the fundamental forces do consume energy when they act, they have to get it from somewhere, it has to go somewhere after the forces have acted, it can't just disappear. Forces don't consume energy, they just transfer it. If gravity is using energy in some way when it pulls a box down against a table, then that energy has to come from somewhere, and go somewhere. Where do you say those places are? Your concept of energy behind the fundamental forces is essentially meaningless without a concept of how that energy actually affects the real world.

So my question to you is: If there really is an energy behind all fundamental forces as you say, how exactly does that energy act on a box sitting on a table? Where does it come from? Where does it go?

 

[gmax137]

I think net force implies work. I cannot think of anything which, when subjected to a net force, does not move, except an infinitely massive object.

Net force does not imply work.

What about the Earth orbiting the sun? The net force is the gravitational attraction. There is motion (the orbit) but there is no work done. Remember 'work is force times distance' is a simplification; the real definition recognizes that force is a vector, and you 'dot product' that with the direction vector. In the orbit the force vector and the direction are always at right angles, so the dot product is zero. There is no work done.

Force, work, and energy are three different concepts, this thread has them all mixed up.

 

[Rap]

Net force does not imply work.

What about the Earth orbiting the sun? The net force is the gravitational attraction. There is motion (the orbit) but there is no work done. Remember 'work is force times distance' is a simplification; the real definition recognizes that force is a vector, and you 'dot product' that with the direction vector. In the orbit the force vector and the direction are always at right angles, so the dot product is zero. There is no work done.

Force, work, and energy are three different concepts, this thread has them all mixed up.

Well I don't think its that bad. We have been (implicitly) assuming a one-dimensional problem, and you are absolutely right, when writing dW=F dx, it should be made clear that F and dx are in the same direction. There is confusion here, and we are trying to clear it up without going too much over the head of the poster of the problem. The problem is now:

A. I observe, in all corners of the universe, forces being created. A star supernovas, creating forces which eject matter. A power plant burns natural gas to create forces which turn turbines. A rocket moves through space in response to exhaust forces. Cars move along roads, powered by engines. And the common theme for each of these forces is that energy was provided to create them, none of them simply sprang into being from nowhere.

B. Taking A's observation, I turn to nature and see forces like gravity.
C. I then quite naturally ask, since A's forces required energy to create, don't B's forces?

So how would you clear up the confusion here without jumping too far beyond falcon32's present knowledge?

 

[Drakkith]

But the force of gravity is being countered by the upward force of the ground on the box. The net force is zero, so there is no work, and so no energy is being converted.

Yes, that's exactly what I was saying.

 

[falcon32]

That's nonsensical/wrong: NONE of the fundamental forces require energy to generate them. You're welcome to continue to refuse to accept this but the universe doesn't care if you do or not.

And I will unilaterally accept your answer, if you stop saying 'that's just the way it is.' Reason, please?

 

[falcon32]

If the fundamental forces do consume energy when they act, they have to get it from somewhere, it has to go somewhere after the forces have acted, it can't just disappear. Forces don't consume energy, they just transfer it. If gravity is using energy in some way when it pulls a box down against a table, then that energy has to come from somewhere, and go somewhere. Where do you say those places are? Your concept of energy behind the fundamental forces is essentially meaningless without a concept of how that energy actually affects the real world.

So my question to you is: If there really is an energy behind all fundamental forces as you say, how exactly does that energy act on a box sitting on a table? Where does it come from? Where does it go?

Grats, you are one out of only two people that replied to my question with some actual insight behind the reply. Thank you for that!

Of course my answer to you is, 'I don't know', and I'm honest enough to admit that. It's all hypothetical, I suppose, and while it may not bother you in the slightest, it bugs the heck out of me!

 

[russ_watters]

And I will unilaterally accept your answer, if you stop saying 'that's just the way it is.' Reason, please?

The universe does not answer "why" questions. Sometimes you just have to accept reality for what it is. Consider this: why aren't you asking "why" it isn't the way you want it to be? You're willing to accept your own preference without a "why" but to overturn your preconception you need a "why". That's a bad attitude for learning how reality works.

 

[Rap]

What do you mean "the rocket is losing mass"?Are you implying the loss of fuels?Well...I didn't go that far!
Just think of any case that an object moves with constant speed.Work is being done while the net force is zero.

Yes, it is losing mass because it is losing burnt fuel.

If an object (with constant mass) is moving at constant speed, then the net force is zero and no work is being done.

 

[douglis]

Yes, it is losing mass because it is losing burnt fuel.

If an object (with constant mass) is moving at constant speed, then the net force is zero and no work is being done.

...but when I lift an object with constant speed(zero net force) I don't do work?

 

[JaredJames]

...but when I lift an object with constant speed(zero net force) I don't do work?

When moving at constant speed all forces are balanced.

Put simply, the 'thrust' you are applying as it moves vertically = the weight of the box.

Because force is a vector with magnitude and direction, work also has this direction.

In the case of your box you have two forces with opposing directions - the work cancels out and gives you a net of no work being done.

 

[douglis]

When moving at constant speed all forces are balanced.

Put simply, the 'thrust' you are applying as it moves vertically = the weight of the box.

Because force is a vector with magnitude and direction, work also has this direction.

In the case of your box you have two forces with opposing directions - the work cancels out and gives you a net of no work being done.

Yes...the net work for the system muscle/object is zero.But the muscles tranfer energy on the object hence they produce work.

 

[JaredJames]

Yes...the net work for the system muscle/object is zero.But the muscles tranfer energy on the object hence they produce work.

The muscles do chemical work, but the mechanical work of the box is zero.

Your hand is doing work on the box, but gravity is doing equal and opposite work on the box. Net result is zero work.

 

[C. H. Fleming]

Your body is not in equilibrium when holding the box up. If the force of your muscles does not exactly balance with the weight of the box, then your muscles will have to compensate. There are short periods of time when the force of your muscles do not exactly balance with the weight of the box. During that compensation phase your muscles will do mechanical work (over some tiny distances). If your body could become perfectly rigid like the table, then this work would not be necessary.

Think about the work necessary to roll a ball up a hill. Now consider balancing the ball upon the side of a hill with nothing more than a rapid series of tiny taps. This costs work. With that same work you could choose to roll the ball up the hill.

Also consider this:

Why do they carry weights on their heads and not with their arms?

 

[mishrashubham]

And I will unilaterally accept your answer, if you stop saying 'that's just the way it is.' Reason, please?

Falcon32, I really appreciate your curiosity and inquisitiveness. I also tend to ask a a lot of 'why' questions and believe me I fallen into trouble because of that many times. But I don't regret it and nor should you. Because that is the spirit that we need in science; asking questions about why the universe works the way it does and not just accepting everything on face value.

I would like to point out one thing however. In science one prominent approach of understanding nature is by analyzing its components. We study larger things by studying the smaller things of which it is made. This is called reductionism. So we say "X" behaves a certain way because the things that it is made of i.e. "A", "B" and "C" behave a certain way. We then divide these into yet smaller parts and attempt to study them. We can keep on doing this, but at some point we will reach a stage where we find that we cannot take out smaller parts. Then we have to accept the fact that "That is how it is because that is how we observe it is".

Same is the case with forces. There are four fundamental forces, namely Gravitational, Electromagnetic, Strong Nuclear and Weak nuclear. All other forces are just macroscopic manifestations of the interactions of these four forces that is to roughly say that they are all made of these four fundamental ones. That is why they are called fundamental.

So let me get this clear; fundamental forces forces do not result from the interaction of energy but instead hold energy in their fields (as potential energy). Rockets on the other hand produce a force from the combustion of fuel which supplies the energy that is needed to do the work when it is lifted from the surface of the Earth against gravity

 

[Rap]

Your body is not in equilibrium when holding the box up. If the force of your muscles does not exactly balance with the weight of the box, then your muscles will have to compensate. There are short periods of time when the force of your muscles do not exactly balance with the weight of the box. During that compensation phase your muscles will do mechanical work (over some tiny distances). If your body could become perfectly rigid like the table, then this work would not be necessary.

The small movements of your muscles do perform work, but it is not necessary. Please see #33 for a case where there is no work done on an object but work must be done (and converted to heat) in order to hold the object motionless in a gravitational field. The muscles in your arms heat up as you hold the box motionless, chemical reactions occur, etc.

 

[C. H. Fleming]

The small movements of your muscles do perform work, but it is not necessary. Please see #33 for a case where there is no work done on an object but work must be done (and converted to heat) in order to hold the object motionless in a gravitational field. The muscles in your arms heat up as you hold the box motionless, chemical reactions occur, etc.

I think that the limit of small fluctuations this is the same principle. Your muscles expend energy during compensation (and do work on the box). Then the box relaxes (and does work on your muscles). However, your muscles do not operate via reversible processes. So that energy is transformed into heat.