Energy conservation in collisions

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Discussion Overview

The discussion centers on the conservation of energy in different types of collisions, specifically elastic and perfectly inelastic collisions. Participants explore the definitions and implications of these terms, as well as the physical mechanisms involved in energy transformation during collisions.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions why energy is conserved in elastic collisions but not in perfectly inelastic collisions, noting a lack of explanation in their textbook.
  • Another participant suggests that the definitions of elastic and inelastic collisions inherently include the conservation of kinetic energy in elastic collisions and its dissipation in inelastic ones.
  • It is noted that energy is always conserved, but kinetic energy specifically is not conserved in inelastic collisions due to the generation of heat, sound, and permanent deformation.
  • A participant points out that elastic collisions can also generate sound, indicating a potential misunderstanding of the definitions provided in the textbook.
  • One participant emphasizes that momentum is conserved in all collisions, while kinetic energy is only conserved in perfectly elastic collisions, and any generation of other forms of energy indicates a non-elastic collision.
  • Another participant reflects on the simplification of the concept of perfectly elastic collisions, suggesting that the textbook may not have clearly distinguished between elastic and perfectly elastic collisions.

Areas of Agreement / Disagreement

Participants generally agree on the definitions of elastic and inelastic collisions, but there is some disagreement regarding the implications and nuances of these definitions, particularly concerning the nature of perfectly elastic collisions and the examples provided in the textbook.

Contextual Notes

There is a noted ambiguity in the textbook's definitions and explanations, leading to confusion among participants about the distinctions between elastic, perfectly elastic, and inelastic collisions. The discussion reveals a reliance on definitions that may not fully capture the complexities of energy conservation in real-world scenarios.

member 392791
I was curious, why is energy conserved in elastic collision but not in perfectly inelastic collision? It said this in my textbook without giving any reason why.
 
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Umm, isn't this the DEFINITION of elastic and inelastic collisions?

If you want a physical intuition beyond that, we think of elastic collisions as those where the two balls colliding are perfectly rigid. So when they hit each other, they just immediately start moving in a different direction, their accelerations being instantaneous. In an inelastic collision, as the balls hit they deform each other before moving off in different directions. This deformation leads to the generation of sound, heat, and frictional forces between the two, all of which dissipate energy. Being perfectly rigid, the hard-sphere collision does not suffer from this.
 
'Energy' is always conserved, it is 'Kinetic energy' that is not conserved in inelastic collisions because of the generation of heat, sound and permanent deformation.
 
ok well elastic collisions generate sound too...I didn't realize this was the definition, the book didn't make it seem like those were the definitions, they just stated that one conserves kinetic energy and the other doesn't, while both conserve momentum, without explaining it, leaving me confused.
 
You are correct to say that momentum is conserved in all collisions. Kinetic energy is only conserved in perfectly elastic collisions. If any other kind of energy is generated then it is not perfectly elastic.
If sound is produced the collision is not perfectly elastic.
I think it is fair to say that perfectly elastic collisions only occur between atoms and molecules.
 
Oh ok, so it's that a ''perfectly elastic'' collision has no sound, heat, or anything else. I think this is a simplification made for intro level, because we are having perfectly elastic collisions applying to more than atoms. I was thinking elastic, didn't realize there was perfectly elastic. It said elastic and perfectly inelastic without mention of perfectly elastic, but I think they mean to say perfectly elastic.
 

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