Work: Isolated & Non-Isolated Systems Difference

In summary, the conversation discusses the concept of isolated and non-isolated systems, as well as external and/or applied forces in physics. The confusion arises from contradictory information in the textbook, including the use of multiple formulas and equations. The key points to understand are that an isolated system is one where each force exists within it, while a non-isolated system has external forces. Work is a transfer of energy, and the author clarifies that setting Ug (potential energy) to 0 in two places is acceptable as it will cancel out in the final calculations. Ultimately, the conversation highlights the complexity of the topic and the need for further clarification.
  • #1
Const@ntine
285
18
Okay, so I've reached the Work Chapters in my textbook, and I've noticed some contradictions, especially in what consists (and what doesn't) an Isolated System, plus the external and/or applied forces.

For example, in one of the "Speed Questions" it categorizes a single cube as a non-isolated system, the surface as a non-isolated system, and the cube/surface (there is friction) as an isolated system. Later, in an exercise, it describes a cube/surface system with friction as a non-isolated system.

The problem is, it's got a ton of formulas that he constructs, reconstructs, renames and whatnot, and it's confused the hell out of me. From what I gathered, an isolated system is something that each force exists within (eg a cube, the surface and the earth), and a non-isolated one is something that each (or some) force(s) is/are external (eg just a cube, where the force that the Earth exerts onto it is external).

Can anybody plainly explain to me the nature of those systems, external/applied forces, and how work fits in all this? I mean, I've got about 50 or so equations and formulas at this point, with most of them lumped together and reconstructed in every page. I've memorized most of the formulas and mostly know how to use them in the various exercises, but I'm kinda lost as to why I'm using them. Things were pretty clear fro my High School studies, and I never had any particular trouble with Work and the like, but this book (Physics for Engineers and Scientists, 8th Edition)has confused me a bit.

I'd really appreciate it if someone could go over the basics briefly.
 
Physics news on Phys.org
  • #2
Darthkostis said:
an isolated system is something that each force exists within (eg a cube, the surface and the earth), and a non-isolated one is something that each (or some) force(s) is/are external
Yes, this is exactly the difference.

Darthkostis said:
hysics for Engineers and Scientists, 8th Edition
By Serway?

Darthkostis said:
external/applied forces, and how work fits in all this?
So work is a transfer of energy. If the forces are internal then work just moves energy around within the system. If a force is external, then energy can leave or enter the system via the external force.
 
  • #3
Dale said:
Yes, this is exactly the difference.

Well, that's good, at least I've got that down.

Dale said:
By Serway?

Yeah.

Dale said:
So work is a transfer of energy. If the forces are internal then work just moves energy around within the system. If a force is external, then energy can leave or enter the system via the external force.

Okay, yeah, I get that. But, for example, in some exercises I see that he sets two different "places" where Ug = 0, one for each object. So, for example, let's say there are two cubes. One is stationed at a reclining surface, and the other is on top of a spring, that is stationed vertically.Both are connected with a weightless rope. We pull the first cube by h, and let him go (Vi = 0). So, when it's time to do the exercise, he says that Ug = 0 in two ocasions: One, when the cube on the reclining surface is dragged/pulled back by h, and two, when the second cube is back at its original place (on top of the spring, which is unstreched).

Can we do that? From high school, I knew that you could just pick one place where Ug was 0. I'd never seen a problem where you could set two places.
 
  • #4
All that matters is differences in potential energy. If you want to set Ug equal to 0 for both at the same time then that is fine too. You will just get a constant on both sides that cancels out. The author is just recognizing that and setting that canceling term to 0 in advance.
 
  • #5
Dale said:
All that matters is differences in potential energy. If you want to set Ug equal to 0 for both at the same time then that is fine too. You will just get a constant on both sides that cancels out. The author is just recognizing that and setting that canceling term to 0 in advance.

So, for each object, I'm free to set Ug zero as I see fit.
 
  • #6
Darthkostis said:
So, for each object, I'm free to set Ug zero as I see fit.
If you ever think that you need to set Ug at some place then don't hesitate to do so. If you were free to set it somewhere else then it will drop out automatically. Personally, I would have set them to the same to be safe. I would have carried an extra term in my intermediate calculations, but I would rather do that than confuse myself
 
  • #7
Dale said:
If you ever think that you need to set Ug at some place then don't hesitate to do so. If you were free to set it somewhere else then it will drop out automatically. Personally, I would have set them to the same to be safe. I would have carried an extra term in my intermediate calculations, but I would rather do that than confuse myself

Okay, thanks for the info! These last few chapters are kinda tricky, but as I progress, through the exercises, things become more clear. Now if only I had more time...
 
  • Like
Likes Dale

Related to Work: Isolated & Non-Isolated Systems Difference

1. What is the difference between an isolated and non-isolated system?

An isolated system is one that does not exchange energy or matter with its surroundings, while a non-isolated system does allow for such exchanges.

2. How are isolated and non-isolated systems used in scientific research?

Isolated systems are often used to study the behavior and properties of closed systems, while non-isolated systems are used to observe the effects of external influences on a system.

3. Can a system switch between being isolated and non-isolated?

Yes, a system can switch between being isolated and non-isolated depending on its surroundings and the conditions of the system itself.

4. Are there any real-world examples of isolated and non-isolated systems?

An example of an isolated system is a sealed thermos, where no heat or matter can enter or escape. An example of a non-isolated system is a plant growing in a garden, where it exchanges energy with the sun and matter with the soil.

5. How do isolated and non-isolated systems relate to the laws of thermodynamics?

The first law of thermodynamics states that energy cannot be created or destroyed, only transferred or converted. Isolated systems follow this law, while non-isolated systems may have energy exchanges with their surroundings. The second law of thermodynamics states that the total entropy of a system and its surroundings always increases in a closed system. This can be observed in both isolated and non-isolated systems as they reach equilibrium and their energy is evenly distributed.

Similar threads

Replies
6
Views
4K
  • Introductory Physics Homework Help
Replies
2
Views
1K
Replies
12
Views
1K
Replies
1
Views
95
  • Mechanics
Replies
30
Views
1K
Replies
3
Views
2K
  • Mechanics
Replies
8
Views
22K
Replies
5
Views
1K
Replies
4
Views
1K
Back
Top