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

[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.

 

[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.