- #1
Miraj Kayastha
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If I push a book horizontally across a table I do work. But is energy conserved?
Is energy conserved in a closed system ?
- Context: High School
- Thread starter Miraj Kayastha
- Start date
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- Tags
- Closed Closed system Energy System
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Discussion Overview
The discussion revolves around the question of whether energy is conserved in a closed system, specifically in the context of pushing a book across a table. Participants explore the implications of work done against friction and the definitions of closed systems, mechanical energy, and energy conservation.
Discussion Character
- Debate/contested
- Conceptual clarification
- Technical explanation
Main Points Raised
- Some participants argue that in a closed system, energy is conserved, while others suggest that in the example of pushing a book, energy is not conserved due to the work done against friction converting some energy into heat.
- One participant emphasizes that energy conservation is a principle of thermodynamics, noting that internal forces and energy transformations can complicate the analysis.
- Another participant points out that if the system is defined poorly or boundaries are not drawn correctly, it may appear that energy is not conserved.
- There is a distinction made between mechanical energy and other forms of energy, with some arguing that mechanical energy is not conserved due to friction, while others maintain that total energy remains conserved when all forms are accounted for.
- Participants acknowledge the possibility of energy escaping as sound, which complicates the conservation analysis.
Areas of Agreement / Disagreement
Participants express differing views on whether energy is conserved in the specific scenario presented. There is no consensus on the definitions of the system or the types of energy considered, leading to multiple competing perspectives.
Contextual Notes
Limitations include varying definitions of closed systems, mechanical energy, and the treatment of heat and sound in energy conservation discussions. The complexity of internal forces and energy transformations is also noted.
Who May Find This Useful
This discussion may be useful for individuals interested in thermodynamics, energy conservation principles, and the nuances of mechanical systems in physics.
- #2
KingCrimson
- 43
- 1
conserved ? in a closed system , such as our universe , or a tightly sealed room that let's in/out absolutely no radiation , yes it is .
in your example , no
because as you push the book , you do work to move the book through the resistance of the table , some of this work is radiated as heat energy
i guess your question is pretty vague , you should tell us more about the system and i can tell you if that system is closed , then yes energy is conserved
in your example , no
because as you push the book , you do work to move the book through the resistance of the table , some of this work is radiated as heat energy
i guess your question is pretty vague , you should tell us more about the system and i can tell you if that system is closed , then yes energy is conserved
- #3
UltrafastPED
Science Advisor
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Yes, energy is conserved when you push the book across the table.
There are several forces here, and the ones that are internal to your system (your muscles and tendons; the detailed work performed in your cells; the increase in heat of your body, etc) are often obscure and are difficult to track.
Conservation of energy is the first law of thermodynamics, and when you properly take into account all of the forces and distances, plus any changes in potential energy, you will find a balance.
But it is much easier to analyze with a purely mechanical system.
There are several forces here, and the ones that are internal to your system (your muscles and tendons; the detailed work performed in your cells; the increase in heat of your body, etc) are often obscure and are difficult to track.
Conservation of energy is the first law of thermodynamics, and when you properly take into account all of the forces and distances, plus any changes in potential energy, you will find a balance.
But it is much easier to analyze with a purely mechanical system.
- #4
CWatters
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In a closed system energy is always conserved. In your example the work you did was turned into heat by the friction between the book and the table. If you could measure that heat you would find it matched the work you did exactly.
If you ever come across an example where energy appears not to be conserved it's likely you have forgotten to include or account for something (In other words you have defined your system badly or drawn your system boundary incorrectly).
Edit: Now that I think about it not all the work you did might have been converted into heat. Some energy may have escaped as sound. See I made a mistake, I didn't draw my system boundary carefully enough :-)
If you ever come across an example where energy appears not to be conserved it's likely you have forgotten to include or account for something (In other words you have defined your system badly or drawn your system boundary incorrectly).
Edit: Now that I think about it not all the work you did might have been converted into heat. Some energy may have escaped as sound. See I made a mistake, I didn't draw my system boundary carefully enough :-)
- #5
Doc Al
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No worries... that sound will eventually end up as 'heat'.CWatters said:
- #6
tiny-tim
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Hi Miraj Kayastha! 
When we ask whether energy is conserved in a particular system, we usually mean mechanical energy (ie KE and PE but not heat or sound etc) …
in this case, no, mechanical energy is not conserved, because friction turns some energy into heat (which "doesn't count").
If there's no friction, then yes, mechanical energy is conserved.
Miraj Kayastha said:
When we ask whether energy is conserved in a particular system, we usually mean mechanical energy (ie KE and PE but not heat or sound etc) …
in this case, no, mechanical energy is not conserved, because friction turns some energy into heat (which "doesn't count").
If there's no friction, then yes, mechanical energy is conserved.
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