Gravity & Energy: Questions on Object Acceleration & Hydrogen Cloud Collapse

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    Energy Gravity
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Discussion Overview

The discussion revolves around questions regarding the acceleration of objects in gravitational fields and the energy dynamics involved in the collapse of a hydrogen cloud into a star. It touches on concepts from both classical Newtonian gravity and General Relativity, exploring the nature of energy and mass in these contexts.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether an object moving towards Earth gains extra energy from the gravitational field and asks where that energy originates.
  • Another participant suggests that the potential energy of the object converts to kinetic energy as it falls, indicating no net change in energy.
  • A similar question is posed regarding a hydrogen cloud collapsing into a star, with the participant wondering if the mass-energy changes during the collapse and where any extra energy comes from.
  • A response indicates that the hydrogen cloud has potential energy due to the separation of hydrogen atoms, asserting again that there is no net change in mass-energy.
  • One participant introduces a perspective from General Relativity, suggesting that energy gain is not observed from a free-falling frame, but questions the implications of energy in that frame, arguing it seems inadequate.

Areas of Agreement / Disagreement

Participants express differing views on the nature of energy changes in gravitational contexts, with some asserting no net change in energy while others challenge this perspective, particularly in relation to General Relativity. The discussion remains unresolved with multiple competing views present.

Contextual Notes

There are limitations regarding assumptions about energy conservation in gravitational fields and the definitions of potential and kinetic energy. The discussion also highlights the complexity of applying General Relativity to these scenarios without reaching a consensus.

BobiG
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I have a quick question, if an object is moving straight towards say Earth, isn't the gravitational field of Earth going to accelerate that object giving it extra energy? Where does that energy come from? Similar question, you have a hydrogen cloud in space, isn't the mass(energy) of that cloud less than the mass(energy) of that same cloud when it collapses into a star due to gravity? Again where does the extra energy come from? It may be a stupid question but I have to ask!
 
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BobiG said:
I have a quick question, if an object is moving straight towards say Earth, isn't the gravitational field of Earth going to accelerate that object giving it extra energy? Where does that energy come from?
If an object "is moving straight towards say Earth", it must have been above the Earth and so has potential energy. That potential energy converts to kinetic energy.
There is no net change in energy.

Similar question, you have a hydrogen cloud in space, isn't the mass(energy) of that cloud less than the mass(energy) of that same cloud when it collapses into a star due to gravity? Again where does the extra energy come from? It may be a stupid question but I have to ask!
Similar answer- the hydrogen cloud has potential energy due to the fact the hydrogen atoms are separate. There is no net change in "mass-energy".
 
Not a stupid question at all. I suppose everyone just goes back to Newtonian gravity to answer that one, since I don't think General Relativity has an answer to it. GR says that the object isn't gaining energy when viewed from the appropriate free-falling frame. I think that's a cop-out, because in that frame, the ground is accelerating toward you. Where does *that* energy come from?
 

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