I don't understand work

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This is a school related question, but it isn't homework, so I wasn't sure where to put it.

Anyway, I'm a sophomore in high school, and I'm required to take physical science, but I don't understand any of it. My teacher doesn't help - she doesn't even understand the questions that I have. I actually wasn't doing so bad (still not great, but not terrible) until we got to the discussion on work. Her entire definition is "force times distance". That's simple enough, and all of my math is correct, but I don't understand what "force times distance" really means. I asked the teacher, and she just said, "That's the way it is". OK, fine. But that doesn't really help me learn it! What exactly is work, and why is it force times distance instead of force times time or force times mass? Please help! I'm so frustrated I want to cry. In fact, I've cried several times in this class.
 

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  • #2
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Work is measured in joules, which is energy. Force times time or force times mass doesn't give you joules. Force times distance does.
 
  • #3
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Uh ... OK? So that tells me what it's measured in, but not what it is.
 
  • #4
I understand your frustration. In fact, I shared it at one point. Believe it or not, even advanced physicists didn't just come out of the womb knowing this stuff. ;-)

As to your question: I'm going to start at the very beginning, since I think that's what you're really after.

When we want to identify an object, we can use all sorts of descriptive terms: It's blue, it's round, it's hot, etc. In physics, we get a little bit more specific. We want to know its mass, its temperature, its density. In other words, we use numbers with units attached to make them meaningful. There's one specific kind of descriptor that tells us how good an object (or a "system") is at causing some kind of change, and that's called energy. Energy is sort of like the "pizzazz" of an object; it's the "get-up-and-go". You can really relate it to your everyday notion of energy: When you drink 6 cups of coffee you really feel like you can take on the world.

Work is what happens when energy enters a system by a force acting through a distance, and therein lies that famous formula. Think about what this means: When you push down on a table, the table doesn't move, and its energy stays the same. When you push on the side of it, however, energy is added: kinetic energy (because now the table is moving). It's added as long as your pushing. When you stop pushing, energy is no longer added, but for a while, it was accumulating. The extra energy that was accumulated (which has to go somewhere, in this case it probably turned into heat) is called work.

Does this begin to answer the question?
 
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  • #5
Think of it this way:
First understand these:
kg = kilograms
m = meters
s = seconds.

Now, force, measured in Newtons (kg/m/s^2) times distance, measured in meters, would give you the new unit of: kg/m^2/s^2 ....

For me it always helps to look at the actual units because then you can see what is happening mathematically :)
Hope this helped a little... and trust me, I feel your pain.
 
  • #6
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I understand your frustration. In fact, I shared it at one point. Believe it or not, even advanced physicists didn't just come out of the womb knowing this stuff. ;-)

As to your question: I'm going to start at the very beginning, since I think that's what you're really after.

When we want to identify an object, we can use all sorts of descriptive terms: It's blue, it's round, it's hot, etc. In physics, we get a little bit more specific. We want to know its mass, its temperature, its density. In other words, we use numbers with units attached to make them meaningful. There's one specific kind of descriptor that tells us how good an object (or a "system") is at causing some kind of change, and that's called energy. Energy is sort of like the "pizzazz" of an object; it's the "get-up-and-go". You can really relate it to your everyday notion of energy: When you drink 6 cups of coffee you really feel like you can take on the world.

Work is what happens when energy enters a system by a force acting through a distance, and therein lies that famous formula. Think about what this means: When you push down on a table, the table doesn't move, and its energy stays the same. When you push on the side of it, however, energy is added: kinetic energy (because now the table is moving). It's added as long as your pushing. When you stop pushing, energy is no longer added, but for a while, it was accumulating. The extra energy that was accumulated (which has to go somewhere, in this case it probably turned into heat) is called work.

Does this begin to answer the question?
It's actually starting to make sense!

A couple more questions might get me through this.

I know you said that energy is added as long as I push the table, so why isn't it force times time?
 
  • #7
It's actually starting to make sense!

A couple more questions might get me through this.

I know you said that energy is added as long as I push the table, so why isn't it force times time?
That was sloppy wording on my part and I apologize (although that would be a nice segue into a conversation about impulse and momentum ...)

Anyway, at the end of the day what you should realize is this: work done is a measure of how much energy is added to an object by a force that causes it to move some distance. This quantity depends on two things: the strength of the force, and the distance it moves.
 
  • #8
russ_watters
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Uh ... OK? So that tells me what it's measured in, but not what it is.
Ultimately, the word "work" is just a name attached to a useful mathematical relationship. force times distance is a useful mathematical relationship. So it is most important to view "work" not in terms of verbal descriptions, but in terms of how force times distance can be used in useful ways. The most obvious is that it useful in the equations of motion and energy, but it is also directly related to heat. That's extremely important in thermodynamics, where you convert heat energy to mechanical work (or vice versa).
 
  • #9
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It's actually starting to make sense!

A couple more questions might get me through this.

I know you said that energy is added as long as I push the table, so why isn't it force times time?
It's not force times time because I've been sitting on my chair for about an hour now, my *** applying considerable force to it the whole time....and yet I feel quite rested. I've been applying force x time to the chair and yet no energy has been used.

When you're sleeping, you're also applying force to the mattress the whole entire time. Look at how much it compresses. Yet you use no energy.

So you see, it is only when you apply a force AND a motion, do you spend energy. This it true even when on the surface it doesn't seem like it. A helicopter sitting still in the air is using up energy...but that's because it's propeller is causing the air to move. When holding a heavy book you might still get tired, but at some level deep in your muslces, some motion is happening even though you might not feel it. Still, it's much easier holding that book still than actually lifting it up in the first place.
 
  • #10
Delta2
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Well force times time is another usefull concept and quantity in physics which is called impulse. Impulse changes the momentum , work changes the energy. Momentum and energy are key concepts in physics and are related and if momentum changes then the energy changes also.

In the example with the table the impulse of the force is not zero in either of the 2 cases (horizontal or vertical push). However the work of the force is zero in vertical push but it isnt zero on horizontal push. The momentum of the table remains zero in vertical push though there is impulse from the force we aplly to it. That is because there is another force coming from the floor where the table is based on, which creates another impulse that cancels out with the impulse of our force. So we see that an impulse needs another negative impulse in order not to change the momentum of an object, however in order not to change the energy we dont need a negative work we just need a negative force to cancel out our force so that the object will NOT move.

Maybe i confused u more since i throw in another pair of concepts (impulse and momentum) but i think the only way to understand more about it and why we define work this way is to read on and try to understand similarities and differences between impulse-momentum, work-energy, impulse-work, and momentum-energy.
 
  • #11
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It's moving day & you are moving into a new apartment. The elevator is broken, so you have to use the stairs. Now you're carrying boxes of books to the new apartment. If the apartment is one flight up, it's not too bad. If it's two flights up, you do twice the *work* carrying the boxes up. Get it?

The force in this case is gravity (the weight of the box). The distance is how high you need to carry the box. Twice as high --> twice the work.
 
  • #12
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It's actually starting to make sense!

A couple more questions might get me through this.

I know you said that energy is added as long as I push the table, so why isn't it force times time?
As Bluesurge stated "When you push down on a table, the table doesn't move, and its energy stays the same"

It is always important to keep in mind that work is a transfer of energy. It does not matter how LONG you push down on the table, if it doesn't move then its energy stays the same. That is why it is not Force X time.

There is a caveat to this: if it does move, then the motion can be written in terms of the time it took to travel a given distance, so work is implicitly dependent on time. However, I won't explain the subtleties of this distinction since it ranges a bit off topic.

And to add to the growing visualize-this list of explanations: Put a weight on one end of a scale, then put a lighter weight on the other end. The lighter weight is pushing down but not enough to move the heavier weight. Thus a force is applied but no work is done. Now exchange the light weight with one heavier than the one on the other side of the scale. Now the applied force cause the other weight to rise and energy is transfered. Namely, as one weight lowers, its potential energy reduces. The other weight rises its potential energy increases. (Plus a kinetic energy term due the overall motion of the system.)

BANG!
 
  • #13
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... There is a caveat to this: if it does move, then the motion can be written in terms of the time it took to travel a given distance, so work is implicitly dependent on time. However, I won't explain the subtleties of this distinction since it ranges a bit off topic....
I'm not sure what you're getting at here. If the OP lifts a 40 lb box of books up a 20 foot flight, s/he has done 40 * 20 = 800 foot-pounds work on the box. Whether s/he does it in 10 seconds or 10 years, it's still 800 ft-lb.
 
  • #14
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I'm not sure what you're getting at here. If the OP lifts a 40 lb box of books up a 20 foot flight, s/he has done 40 * 20 = 800 foot-pounds work on the box. Whether s/he does it in 10 seconds or 10 years, it's still 800 ft-lb.
Ever heard of Power?

BANG!
 
  • #15
Redbelly98
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(Thread title) said:
I don't understand work
The short answer I tell people is: doing work on an object is how we change it's kinetic energy.

Here I am talking about the total (or net) work done by the net force acting on an object. As long as the object moves while a nonzero net force acts on it, it will either speed up or slow down (have a change in its kinetic energy).

We can also talk about the work done by individual forces, even if the net force does zero work.
 
  • #16
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Wow! You guys are very helpful and nice. Thanks! I think I understand it much better now. I'll be sure to come back when (not if: when) I have more questions.
 
  • #17
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Not to keep beating the horse, but I made up this little cheat sheet to try to lodge the force/work/power relationships in my own mind:
http://www.etantdonnes.com/MACHINE/TABLES/energy.txt

Every-measurable-thing can be reduced to distance, mass, time, and a few other basic quantities (I'm equivocating because I don't want to mis-state one of the quantities). If you have some measurement that you don't understand, it usually helps to reduce it to those basics. And if you have some relationship or function that you don't understand, trying to figure out its quantities is a good first step. In fact, recently someone on this list pointed out that the famous E=MC^2 formula is "obvious" (ha ha) because energy is measured as:
Kg x (M/S)^2
 
  • #18
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Ever heard of Power?

BANG!
I know what 'power' is, and maybe you know, but the OP doesn't (or didn't) know what 'work' is. I think it's best to get that concept firmly understood before attempting to discuss power. One step at a time, OK?

There is a caveat to this: if it does move, then the motion can be written in terms of the time it took to travel a given distance, so work is implicitly dependent on time.
OK - I lift 40 pound weight up 20 feet. Show me how this motion can be written in terms of the time it took.
 
  • #19
Not to keep beating the horse, but I made up this little cheat sheet to try to lodge the force/work/power relationships in my own mind:
http://www.etantdonnes.com/MACHINE/TABLES/energy.txt

Every-measurable-thing can be reduced to distance, mass, time, and a few other basic quantities (I'm equivocating because I don't want to mis-state one of the quantities). If you have some measurement that you don't understand, it usually helps to reduce it to those basics. And if you have some relationship or function that you don't understand, trying to figure out its quantities is a good first step. In fact, recently someone on this list pointed out that the famous E=MC^2 formula is "obvious" (ha ha) because energy is measured as:
Kg x (M/S)^2
Dude that is sweet.
 
  • #21
"it's the most frustrating situation a student could ever have, asking the teacher that something bothers him and the teacher would just answer, 'it's the way it is.' and i understand that. come to think of it, it's your chance to discover new things with yourself alone and that's the beauty of science [most especially the physical sciences]. just put work into this way, work is done whenever an external force "F" [force vector] is applied on something and it covers a distance "s" [displacement vector] with a certain velocity. but remember this, there would be no work if the or in other words ZERO work if F is perpendicular to s which means that you can only attain the maximum work if F is parallel to s. so, in other words the value of work is dependent not only on the force exerted and the distance covered but also the angle between them. by definition of the magnitude of work:

W=Fscos(teta)

where teta is the angle between F and s. the physical meaning of this one, is that work W is just a shadow of F on s and it is in the direction of the displacement vector s. and also take note, work is a scalar quantity. so it can, be of negative value if the the direction of the force vector is opposite of that the displacement vector or it may be caused by the angle between these vectors."

"just don't stop asking questions, for these questions may not be that significant as of today but it could be of more importance tomorrow."

-R. Laride
 
  • #22
rcgldr
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You guys are very helpful and nice. Thanks! I think I understand it much better now. I'll be sure to come back when (not if: when) I have more questions.
Not sure if the OP is still reading this thread.

One issue I have is that although mechanical work can be calculated as force x distance, it's not a good definition for work in general. Work is related to the amount of energy change during some process. It can be mechanical, eletrical, thermal, chemical (bond energy), ....

For examples of various types energy:

http://en.wikipedia.org/wiki/Energy
 
  • #23
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Nice, that you has started to comprehend, I was just studying Work the past week though:D

If you have questions just ask, but I haven't yet finished studying it. Need to learn Newton's laws to continue with work as in some cases we have gravity acting on objects which is the force applied.
 
  • #25
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btw, you still reading this thread ?

And the best thing is to never stop asking questions. That's the best thing to do in the physical sciences. ( even if your teacher does not like it:wink: )

Never rely on your teachers to tell you to learn or to teach you something, better try learn it from yourself and when you have questions just come here:smile:
 

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