Laws of conservation of momentum?

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Discussion Overview

The discussion revolves around the laws of conservation of momentum, specifically addressing whether certain topics—energy lost due to impact, inelastic impact, and purely elastic impact—fall under this principle. The scope includes conceptual clarifications and technical explanations related to momentum and energy conservation during collisions.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants assert that momentum is conserved in all three scenarios as long as no external forces are involved.
  • Others clarify that while momentum is conserved during collisions (both elastic and inelastic), kinetic energy is not conserved in inelastic collisions, as some energy is transformed into heat or sound.
  • There is a distinction made between kinetic energy and total energy, with some participants emphasizing that potential energy due to deformation is also not conserved in inelastic collisions.
  • A later reply suggests that the phrasing regarding energy conservation could be confusing for students, advocating for a more nuanced explanation that includes qualifiers about local conditions.
  • Concerns are raised about students misunderstanding the distinction between kinetic energy not being conserved and energy in general not being conserved.

Areas of Agreement / Disagreement

Participants generally agree that momentum is conserved in the discussed scenarios, but there is disagreement regarding the conservation of energy, particularly kinetic and potential energy, leading to a nuanced discussion about the implications for students' understanding.

Contextual Notes

Limitations include the potential for misunderstanding the distinction between different forms of energy conservation and the need for careful phrasing to avoid confusion among students.

Benjamin_harsh
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Problem Statement: Are this 3 topics comes under laws of conservation of momentum?
Relevant Equations: Are this 3 topics comes under laws of conservation of momentum?

Are this 3 topics comes under laws of conservation of momentum: energy lost due to impact, inelastic impact, purely elastic impact?

[Moderator's note: Moved from a homework forum.]
 
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Yes. Momentum is conserved, so as long as there are no external forces involved, that is true for all three.
 
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Benjamin_harsh said:
Are this 3 topics comes under laws of conservation of momentum: energy lost due to impact, inelastic impact, purely elastic impact?
Are you asking whether the law of momentum conservation is valid in these cases? If yes, then the answer is also yes. During collision, elastic or inelastic, momentum is always conserved. In inelastic case, energy is not conserved in the sense that some amount of the initial energy is lost in the form of heat or sound energy.
 
Wrichik Basu said:
energy is not conserved
Kinetic energy is not conserved. Important distinction.
 
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Orodruin said:
Kinetic energy is not conserved. Important distinction.
Any Potential energy stored due to the deformation of objects is not conserved either. (Hysteresis is at work)
 
Those answers are correct, but they can be confusing to students who are shaky on energy conservation. Would it be better to say that "On Earth, energy is always conserved, but in this scenario some kinetic energy and potential energy can be transformed to heat energy."?

The "on Earth" or "locally" qualifier is needed because energy is not necessarily conserved in the whole universe.

I just worry that a B level student may be sloppy and read, "Kinetic energy is not conserved" as "Energy is not conserved."
 
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anorlunda said:
I just worry that a B level student may be sloppy and read, "Kinetic energy is not conserved" as "Energy is not conserved."
The important distinction is between mechanical energy and thermal energy. Students should understand that "Mechanical energy is not conserved" except under the most ideal of conditions.
 

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