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

[Drakkith]

I agree with your last point, that work requires energy. But paradoxically, work cannot happen without a force. Try moving a rocket without one.

Now your question to me is a very good one. "If it took energy to make gravity work, where is the energy coming from?" Just because I can't answer it (and certainly I cannot), doesn't mean we get to conclude that the forces just exist just because. That is borderline religious. What we should do is try to find out why they do exist, and that may be a task for a future generation.

I appreciate the debate, thanks!

Anytime.

 

[douglis]

But suddenly, when the forces DO cancel, we all shake our heads and say, "no energy here". Shouldn't we rephrase our statement to "no net energy here?"

No work done doesn't mean no energy spent.It just means no energy is transferred(from the muscles to the box).
In other words your muscles contract with zero efficiency.

 

[Rap]

The amount of mechanical work you are doing on the box is zero, because it isn't moving.

What is happening to your body when you get tired has nothing much to do with Newtonian mechanics. Certainly there are a lot of chemical reactions going on in your body, which are consuming food and producing heat and electricity (in your nerves, brain, etc), whatever you are doing. If you hold the box above your head for a long time, some of those chemical reactions get out of balance, the "waste" chemicals build up in your muscles, you feel pain, and you take some action to relieve it (letting go of the box, for example).

Humans aren't very well adapted to holding something above one's head for a long time. If you supported the weight of the box on your head (which is the normal way that people in some cultures carry heavy loads) or on your shoulders, you would be able to carry it in that position for much longer without getting tired, because the weight is then being supported by your bones, not just by the muscles in your arms.

Yes - that's it. Work is being done, but not on the box.

Here is a simple model - suppose you want to hold up an iron cylinder instead of a box. You put it inside of a coil of wire and connect the ends of the wire to a battery. The current will generate a magnetic field which will magnetize the iron bar in the opposite direction, and the bar will be attracted to the coil. If there is enough current in the coil, it will hold the bar up against the force of gravity. There is then no work being done on the bar, but there will be resistance in the coil, and the battery will be doing work to overcome that resistance. The rate of work will be I^2 R where I is the current, R is the resistance of the coil. The work being done is due to the force required to move the electrons through the resistive coil. The work is being converted to heat. When the battery runs down, the iron bar drops.

I think in the case of muscles, electrons are being moved around (although the force is not magnetic) and the work being done to move them is then converted to heat. When the energy source runs out, the box will drop.

 

[Subductionzon]

Rap, I am not really fond of your analogy, though it does show how you can consume energy without doing work.

When holding a box over your head the key is that you are not holding it still. You are always slightly tensing and relaxing your muscles, resulting in very small movement of the box. You can minimize the work you do, and therefore extend the time period that you can hold the box up by perfecting your technique either locking your joints, or if you have the ability, like some animals, to actually locking your muscles. I have seen birds that have died on electric lines when they bumped up against a conductor. Even after they have been dead a day or more you can see them hanging upside down by their feet. The birds muscles can "lock" and very little energy is used to keep it in place.

 

[Rap]

Rap, I am not really fond of your analogy, though it does show how you can consume energy without doing work.

When holding a box over your head the key is that you are not holding it still. You are always slightly tensing and relaxing your muscles, resulting in very small movement of the box. You can minimize the work you do, and therefore extend the time period that you can hold the box up by perfecting your technique either locking your joints, or if you have the ability, like some animals, to actually locking your muscles. I have seen birds that have died on electric lines when they bumped up against a conductor. Even after they have been dead a day or more you can see them hanging upside down by their feet. The birds muscles can "lock" and very little energy is used to keep it in place.

Yes, I agree, if you can somehow use your body to act as a support, by placing it on your head, for example, then you will not expend energy to keep the box above your head. But I thought the point of the OP was that energy WAS being expended to hold the box over your head, and with flexible arm joints, there was no locking effect, and the question was where is the energy going?

Small movements of your arms do no net work on the box. If you drop the box a bit, then push it back to its original position, the net work done on the box is zero. If the muscles in your arm return to their original position, then no work has been done on your arms. Nevertheless, energy has been expended and turned into heat during this process. If you have to think in terms of work, then it will be microscopic, like the electrons in the wire in the bar and coil example. In the case of your arm, it's stored chemical energy being transformed into heat, I'm not sure its helpful to think of this in terms of work being done.

 

[Lsos]

Ludicrous. Of course forces require energy. Observe when a rocket accelerates through space according to F=m*a. W=F*d after the rocket has moved a certain distance, so obviously a force is performing work on the rocket, moving it. Now we all know that energy is the ability to do work. If the force moving the rocket did not possesses energy, it could not have performed work. Any force requires energy. To deny this would be to embrace mental insanity.

When a force creates motion, then you need energy. Since the thrust of your rocket is causing it to move, then of course energy is expended.

When that same rocket sits on the launch pad, there are still some heavy forces acting on it. Yet energy is NOT expanded, because the forces are not causing it to move.

 

[thegreenlaser]

I agree with your last point, that work requires energy. But paradoxically, work cannot happen without a force. Try moving a rocket without one.

Yes, but a force can happen without work being done. What you're saying here is equivalent to "A black lab is a dog, therefore all dogs are black labs." A rocket and an object in static equilibrium are two different things. In the case of the rocket, force is being applied, which creates work; thus energy was required. The key distinction that you're missing is that the energy is a requirement of the WORK, not the force. You're making a jump in logic when you say "Work implies energy and work implies force, therefore force implies energy" because the presence of a force does not require that work be done. You're taking one case (the moving rocket) and assuming that that applies every time a force exists, which is not true.

 

[Rap]

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.

 

[douglis]

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.

If the net force is different than zero the object is accelerated and if the net force is zero the object either moves with constant speed or it doesn't move.Only in the third case no work is done.
Am I right?

 

[Rap]

If the net force is different than zero the object is accelerated and if the net force is zero the object either moves with constant speed or it doesn't move.Only in the third case no work is done.
Am I right?

No, second and third case are zero work. If you have a net force F and the object moved dx under the influence of that force, then the work done on the object is equal to F*dx. If the net force is zero but the object moved - no work. If there is a net force, but the distance moved is zero - no work. I cannot think of a situation where there is a net force but no motion.

 

[thegreenlaser]

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.

Again, you're right; but again you're taking a jump in logic by saying that that supports your argument that energy is expended by the forces on a stationary object. What you said is correct: if an object is subjected to a net or unbalanced force, it will move. Then work will have been done on the object, which is also equal to the energy expended. Great. The problem is, that's pretty much irrelevant to the point you're arguing. I said that work implies force, but force does not imply work. You countered by adding the word "net" in front of the force, which is a completely different argument.

Your argument only holds true for net, or imbalanced forces, like in the case of a rocket. In the case of a stationary box, the forces are balanced, (i.e. \vec{F_{net}} = 0), but that doesn't mean there are no forces involved. There is still force (gravitational force pulling down, and an equal supporting force pushing up) but the net force is 0, so the object will not move. In that case, no work is done, because the object has not moved at all (use your F*d formula for an unmoving object if you don't believe me). There are tons of cases like this. (Ask any engineer taking a statics class, which I am right now). In the case where the NET force equals zero (again, this is NOT equivalent to saying there is no force applied), there is no mechanical work done, and therefore there is no energy transfer.

Edit: Oops, I thought that was the OP replying. Still, my point stands. You can't use work done by imbalanced forces to argue that work is done by balanced forces. Work implies force (imbalanced force, in fact), but force, particularly balanced force, does not imply work. Balanced forces is what we're talking about with an unmoving box, not imbalanced forces. The argument that imbalanced forces create work is true but irrelevant in this case.

 

[JaredJames]

No, second and third case are zero work. If you have a net force F and the object moved dx under the influence of that force, then the work done on the object is equal to F*dx. If the net force is zero but the object moved - no work. If there is a net force, but the distance moved is zero - no work. I cannot think of a situation where there is a net force but no motion.

When two opposing forces have a net force of zero, you get no motion.

 

[douglis]

When two opposing forces have a net force of zero, you get no motion.

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.

If the net force is zero but the object moved - no work.

...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?

 

[falcon32]

This time it is the OP replying

Yes, but a force can happen without work being done.

Agreed.

You're making a jump in logic when you say "Work implies energy and work implies force, therefore force implies energy" because the presence of a force does not require that work be done.

Hmmm. Again, agreed. Just because a force exists does not mean work is going to be done. But is energy expended? How are you to know, or not know, that some universal energy is at work to provide the forces of nature? You cannot disprove it.

You're taking one case (the moving rocket) and assuming that that applies every time a force exists, which is not true.

So take any other instance where mankind creates a force. From cars, to coal plants, to submarines. Did it require energy, or no?

 

[falcon32]

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?

Actually he was incorrect. No object can move under a balanced force situation. The force of the hot gas exhaust which cause the rocket to move in the opposite direction is indeed larger than the downward force of gravity. I believe he might have meant 'there is no acceleration.'

 

[JaredJames]

No object can move under a balanced force situation.

Of course it can.

A non-accelerating vehicle traveling at constant velocity has all forces balanced.

The force of the hot gas exhaust is indeed larger than the downward force of gravity. I believe he might have meant 'there is no acceleration.'

Incorrect. The force downwards equals gravity plus air resistance and so the rocket moves at constant velocity.

No, I did not mean there is no acceleration.

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.

If I push on either side of a box with equal force the net force is zero and there is no motion.

In my example I was referring to a stationary object, I should have made that clear. If an object isn't moving and the net force on it is zero from two opposing forces then it will not move - it was an example of when net force of zero can give no motion.

But as above, at the same time a net force of zero can also occur under motion. The difference is that in the rocket case work is being done and in the stationary box case no work is being done.

 

[douglis]

Actually he was incorrect. No object can move under a balanced force situation. The force of the hot gas exhaust which cause the rocket to move in the opposite direction is indeed larger than the downward force of gravity. I believe he might have meant 'there is no acceleration.'

No,that's wrong.If the hot gas exhaust is larger than the downward force of gravity the rocket will accelerate upwards.If it's the same the rocket will keep moving with constant speed.

 

[falcon32]

Of course it can.

A non-accelerating vehicle traveling at constant velocity has all forces balanced.

Absolutely true. But here's where you make your logical error:
We know that acceleration is a change in velocity.
Did the rocket's velocity change from zero to some real value in the upward direction?
Then, did the rocket experience an acceleration?
If the rocket experienced an acceleration, how were the forces balanced?

Only after the rocket has attained some constant velocity x can we say that the forces are now balanced.

 

[falcon32]

No,that's wrong.If the hot gas exhaust is larger than the downward force of gravity the rocket will accelerate upwards.If it's the same the rocket will keep moving with constant speed.

Right, only when the rocket is traveling at a constant velocity. Not during launch.

 

[douglis]

If I push on either side of a box with equal force the net force is zero and there is no motion.

In my example I was referring to a stationary object, I should have made that clear. If an object isn't moving and the net force on it is zero from two opposing forces then it will not move - it was an example of when net force of zero can give no motion.

But as above, at the same time a net force of zero can also occur under motion. The difference is that in the rocket case work is being done and in the stationary box case no work is being done.

Yes,I was answering to Rap who said "If the net force is zero but the object moved - no work."
I believe he was wrong as I showed in the constant speed rocket example where work is done.

 

[JaredJames]

Right, only when the rocket is traveling at a constant velocity. Not during launch.

The forces aren't balanced during initial launch, nobody mentioned initial launch, we specifically mentioned balanced forces.

So your whole "during launch" point is non-sense.

Only for a short period during the launch are the forces unbalanced. The rocket stops accelerating after a certain point and is traveling at constant velocity with the force of thrust and wind resistance + weight balanced.

 

[JaredJames]

Absolutely true. But here's where you make your logical error:
We know that acceleration is a change in velocity.
Did the rocket's velocity change from zero to some real value in the upward direction?
Then, did the rocket experience an acceleration?
If the rocket experienced an acceleration, how were the forces balanced?

Only after the rocket has attained some constant velocity x can we say that the forces are now balanced.

There is no logical error.

We are talking about a point where forces are balanced, not where you are accelerating.

You are assuming the rocket is constantly accelerating during the launch sequence - it is not.

RE Bolded: That is what we are discussing.

 

[falcon32]

The forces aren't balanced during initial launch, nobody mentioned initial launch, we specifically mentioned balanced forces.

So your whole "during launch" point is non-sense.

Only for a short period during the launch are the forces unbalanced. The rocket stops accelerating after a certain point and is traveling at constant velocity with the force of thrust and wind resistance + weight balanced.

Ok, I agree with you. for some reason I thought initial launch was part of the question. :)

 

[falcon32]

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!

 

[Drakkith]

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!

At some point you are just going to have to accept the fact that this is just the way it works. The 4 fundamental forces of nature do NOT require energy to function. In fact, in general, they RELEASE energy when they do work. The chemical reactions required to provide thrust to a rocket release energy because the end products of that reaction are at a lower energy level than they were before. The energy released is harnessed and used to propel the rocket. Fusing deuterium together releases energy because the resulting nucleus has less mass/energy than the individual components did before they fused.

To me, it looks like that you must only use energy to do work when you want to work AGAINST these forces. Moving something up against gravity, or pulling an electron away from a proton requires an input of energy. So does moving heat from a cooler area to a warmer area, which is the reverse of the way it wants to work, so you have to use energy to do it. Even pushing a box you have to work against the inertia of the object that makes it want to stay just the way it was.

 

[Drakkith]

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.

The forces that released the energy in your examples were the fundamental forces. If they required energy to work, we wouldn't have any extra energy to do anything with.

 

[falcon32]

The forces that released the energy in your examples were the fundamental forces. If they required energy to work, we wouldn't have any extra energy to do anything with.

Not sure I follow you. Supposing for a moment that the answer to my question is 'yes', they require energy, then I would know they are receiving it from somewhere. I wouldn't know from where, but I would know they would have to, since energy cannot be created or destroyed.

Would you agree with that hypothesis?

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.

 

[Drakkith]

All I'm saying is, why don't they required 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.

And you have given me another disappointing "Well, why is it like that" response. Why does mass attract other mass through gravity? Because they bend spacetime around them which results in a curved spacetime that they move through? Well why do they do that? Because they just do. That is what we have observed to happen. It is something you are just going to have to accept.

Why is the electric charge of a proton and an electron exactly equal when the electron is so much less massive than a proton? Because it just is.

Get it?

 

[falcon32]

And you have given me another disappointing "Well, why is it like that" response. Why does mass attract other mass through gravity? Because they bend spacetime around them which results in a curved spacetime that they move through? Well why do they do that? Because they just do. That is what we have observed to happen. It is something you are just going to have to accept.

Why is the electric charge of a proton and an electron exactly equal when the electron is so much less massive than a proton? Because it just is.

Get it?

So just say you don't know, but don't get mad at me for asking questions. In all probability, my question will be answered when we finally discover the correct grand unification theory.

 

[Drakkith]

So just say you don't know, but don't get mad at me for asking questions. In all probability, my question will be answered when we finally discover the correct grand unification theory.

There is never an end to the chain of "Why?" questions. That's my point.