Help -- I don't understand work

[alkaspeltzar]

Work is basically energy applied thru a force to move an object right?

Or another way put it is just force thru a displacement, which can be thought of as "energy used"

I see definitions saying it is a transfer of energy, not sure why they mean by that

 

[kuruman]

By doing work, a force changes energy from one form to another. For example, by doing positive work on a mass falling down, gravity converts gravitational potential energy into kinetic energy and the forces associated with air resistance convert some of the kinetic energy of the mass into increased kinetic energy of the air molecules, the latter being a form of heat.

 

[A.T.]

Can anyone give me a layman's definition of work?

Looks like those "layman's definitions" is what's confusing you.

 

[alkaspeltzar]

Okay but then why is work same as energy? Everything I read yes explains the conversion part, but isn't the force thru the distance like energy or an applied energy

 

[alkaspeltzar]

Looks like those "layman's definitions" is what's confusing you.

How would you define it exactly?

 

[A.T.]

How would you define it exactly?

https://en.wikipedia.org/wiki/Work_(physics)#Mathematical_calculation

 

[alkaspeltzar]

You know, I have looked at that. In fact, if you look at the definition of joule, it has a definition similar to what I posted above. But in the end, links to Wikipedia aren't helping

When your confused your confused. So I was looking for someone to explain how they think about it. Clearly work is a form of energy. It has units of it. I just want to know if work can therefore be thought of as "energy being exerted or expended" on a system. is that what work is? Seems confusion how work and energy have same units.

 

[A.T.]

So I was looking for someone to explain how they think about it.

And when you get two different answers from two different people, then you will be confused again?

Look up some examples how work is used in solving problems. Solve some yourself. Then you will develop your own way to think about it.

 

[hutchphd]

When in doubt, spend time with Feynman:
http://www.feynmanlectures.caltech.edu/I_04.html

 

[Drakkith]

I like to think of energy as a property of an object/system of objects, while work is something that is done to an object or system of objects. It is more like a process, while energy is a property.

Clearly work is a form of energy.

I wouldn't agree.

I just want to know if work can therefore be thought of as "energy being exerted or expended" on a system. is that what work is? Seems confusion how work and energy have same units.

As I said, I like to think of work as a process that adds or subtracts energy from a system. So it makes sense for work to have the same unit as energy.

Okay but then why is work same as energy? Everything I read yes explains the conversion part, but isn't the force thru the distance like energy or an applied energy

Not really. Look at all the forms of energy. I don't believe any of them are defined in terms of a force times a distance.

 

[russ_watters]

Okay but then why is work same as energy? Everything I read yes explains the conversion part, but isn't the force thru the distance like energy or an applied energy

It's not; work is a mechanical transfer of energy. You can also transfer energy via radiation/heat, electricity, chemical transport, etc.

 

[DaveE]

I can see why you are confused. The units are the same (Joules, for example).
I like to thing of Energy as something stored, a property of an object. Work is the amount of Energy that has been transferred or changed by a process. Energy is more static, work involves the amount of energy change. I would say if the Energy isn't changing, then there is no Work. Perhaps like the difference between the dollars in your bank balance versus the dollars that you deposit with your paycheck.
Ultimately it will be the context that matters. One of the nice things about the physical sciences is that you can always go back to the equations. Perhaps for some problem you may refer to Energy and I may refer to Work, but we can both call them Joules and then discuss what is happening to the Joules and how many there are.

 

[alkaspeltzar]

Well, kinetic work energy theorem links works directly to energy. So I do agree that are different as energy is stored but then work is still a "energy applied" or transferred in a sense. If they weren't equal, then conservation of energy wouldn't work either, because work in has to equal something out

 

[Drakkith]

Work is the amount of Energy that has been transferred or changed by a process. Energy is more static, work involves the amount of energy change.

Hmm. Wouldn't the amount of energy transferred just be the amount of energy itself? To me, this makes it seem like there are two names for the same thing, whereas thinking of work as a process in and of itself removes this conflict.

 

[phinds]

Think about this: you pick up a box weighing 100lbs. You carry the box 5 feet and place it on a shelf that is 3 feet higher than the level it was at when you picked it up. You have done a well defined amount of work.

Now pick up that same box and carry it half a mile out and then half a mile back and then place it on the same shelf. You have done exactly the same amount of work as in the first instance.

Do you seriously believe that you have expended the same amount of energy in both cases?

 

[gmax137]

clearly work is a form of energy. It has units of it. I just want to know if work can therefore be thought of as "energy being exerted or expended" on a system. is that what work is? Seems confusion how work and energy have same units.

Careful there. Looking at the units or dimensionality can provide insight but it can also lead you astray.

Wait until you get to torque -- that might "really bake your noodle" -- it too has units force x distance, Newton-meters. but nobody calls out torque in joules.

 

[gmax137]

Think about this: you pick up a box weighing 100lbs. You carry the box 5 feet and place it on a shelf that is 3 feet higher than the level it was at when you picked it up. You have done a well defined amount of work.

Now pick up that same box and carry it half a mile out and then half a mile back and then place it on the same shelf. You have done exactly the same amount of work as in the first instance.

Do you seriously believe that you have expended the same amount of energy in both cases?

Oh dear. Intuitions based on body physiology cause way more misunderstandings than they cure.

 

[Drakkith]

Think about this: you pick up a box weighing 100lbs. You carry the box 5 feet and place it on a shelf that is 3 feet higher than the level it was at when you picked it up. You have done a well defined amount of work.

Now pick up that same box and carry it half a mile out and then half a mile back and then place it on the same shelf. You have done exactly the same amount of work as in the first instance.

Do you seriously believe that you have expended the same amount of energy in both cases?

I'm with gmax. This is more about biology than physics, and will be confusing out of context.

 

[phinds]

I'm with gmax. This is more about biology than physics, and will be confusing out of context.

I don't see why. You expend a lot of energy just using your muscles, but you aren't necessarily getting any work done.

 

[Drakkith]

I don't see why. You expend a lot of energy just using your muscles, but you aren't necessarily getting any work done.

You're doing all that work on your muscles, not on the box. Which just brings the question back to what the difference between energy and work is.

 

[DaveE]

Think about this: you pick up a box weighing 100lbs. You carry the box 5 feet and place it on a shelf that is 3 feet higher than the level it was at when you picked it up. You have done a well defined amount of work.

Now pick up that same box and carry it half a mile out and then half a mile back and then place it on the same shelf. You have done exactly the same amount of work as in the first instance.

Do you seriously believe that you have expended the same amount of energy in both cases?

I can't decide if this is completely irrelevant or just unnecessarily confusing. Best to consider closed systems if you are going to tally up work done vs. energy stored.
I think the discussion of the definition of work is best done with adding poorly defined or unmeasured processes like metabolism, oxygen consumption, or temperature change of human body tissues.
However, if your human porter is 100% efficient, then yes, you will have done the same amount of work.

 

[DaveE]

Hmm. Wouldn't the amount of energy transferred just be the amount of energy itself? To me, this makes it seem like there are two names for the same thing, whereas thinking of work as a process in and of itself removes this conflict.

Hmm. I thought that was what I said. I actually think we all agree about this, we are just describing it differently.

1) Work is not a process, work is a property of the process. Other properties might be force, duration, impulse etc.
2) The amount of energy is also a property, of a thing or, perhaps a process. A flywheel can have energy (stored) when there is no work involved at a specific time. However, the energy of the flywheel can not be changed without work. There must be some process, with work, to change the energy of the object. The energy added by the process is not quantitatively the same as the (total) energy of the object.
3) I know a lot of people will disagree with me, but I do actually think they are nearly the same thing. Energy is a property of things, and it can change when it is moved to other things. We call that change Work (perhaps to confuse 1st year physics students).

I think the OP's question is a good one, because a whole bunch of people who do understand work and energy keep posting different versions of basically the same answer (like me, LOL). However, some add unnecessary confusing bits, undoubtably unintentionally.

 

[A.T.]

So I do agree that are different as energy is stored but then work is still a "energy applied" or transferred in a sense.

If you like this way of thinking, there is an analogy between work and impulse, in that both represent a transfer of a conserved quantity:

Work is to energy like impulse is to momentum

Work is force integrated over distance.
Impulse is force integrated over time.

If they weren't equal, then conservation of energy wouldn't work either, because work in has to equal something out

That's where the analogy breaks down a bit, because there is only one form of momentum, so every impulse has some equal but opposite impulse in a closed system. But since energy can be converted into/from other forms, the negative work doesn't always equal some positive work done.

 

[Drakkith]

3) I know a lot of people will disagree with me, but I do actually think they are nearly the same thing. Energy is a property of things, and it can change when it is moved to other things. We call that change Work (perhaps to confuse 1st year physics students).

I don't agree. We call that change in energy a change in energy, or ΔE commonly in the underlying math. It's a bit like saying that the act of pouring water from a pitcher into a glass is the same thing as the water. The act of pouring the water changed the amount of water by, say, 1/2 liter, so I could use liters as my unit for the act of pouring and as the amount of water in the glass. But you and I both know that the water and the act of pouring the water are totally different things.

2) The amount of energy is also a property, of a thing or, perhaps a process. A flywheel can have energy (stored) when there is no work involved at a specific time. However, the energy of the flywheel can not be changed without work. There must be some process, with work, to change the energy of the object. The energy added by the process is not quantitatively the same as the (total) energy of the object.

But that process itself is all that is needed to change the energy of the object. If the work isn't the process, then the word 'work' is meaningless here.

 

[A.T.]

Energy is a property of things, and it can change when it is moved to other things. We call that change Work

I think "transfer of energy" is better than "change of energy" as description of work done by a force. An individual force can be doing work, while the energy doesn't change, because other forces do negative work.

 

[A.T.]

If the work isn't the process, then the word 'work' is meaningless here.

"Doing work" is the process
"Work" is the amount of energy transferred in the process.

 

[phinds]

You're doing all that work on your muscles, not on the box. Which just brings the question back to what the difference between energy and work is.

Fair enough

 

[tkyoung75]

Work is a form of energy transfer. Heat is the other form of energy transfer.
People used to believe that heat was a fluid, called caloric. This was proven to not be the case through the work of Clausius, Joule, Coriolis, etc. Their work led to the understanding of 'energy' that we have today.
Energy is a property.
Work and heat are both dependent on path, that is to say, the work done in lifting a box three feet is not the same as lifting it three feet, and running around the oval. This is because of inefficiency. There is no such thing as a perfectly efficient process. That leads to entropy.
The theory is developed empirically, that is why it can be confusing.

 

[sophiecentaur]

Wait until you get to torque -- that might "really bake your noodle" -- it too has units force x distance, Newton-meters. but nobody calls out torque in joules.

This is the one time that I think Imperial Units are more helpful than SI and other metric systems. There is a clear distinction (marked on the sides of instruments actually) between Pound Feet (torque) and Foot Pounds (Work). Of course, the Joule is work done when a force of One Newton MOVES THROUGH one metre but it is never called a Newton metre. It would have helped if the unit for torque were chosen to be the metre Newton .

But, in general, the fact that you get the same numerical answer for torque and for work means nothing. We don't expect 2X2 = 4 to apply only in one context so why should it apply here?

@alkaspeltzar : Look at @A.T. 's post #8 again. Doing what he suggests will achieve much more for you than reading all the comments on this thread ten times.

 

[gmax137]

@alkaspeltzar : Look at @A.T. 's post #8 again. Doing what he suggests will achieve much more for you than reading all the comments on this thread ten times.

Plus one! this is really sound advice