Gravitational Potential Energy of an object of mass

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Discussion Overview

The discussion revolves around the gravitational potential energy (GPE) formula, specifically the interpretation of the variable 'r' in the equation GPE = -GM/r. Participants explore whether 'r' refers to the distance from the center or the surface of a massive object and clarify the conditions under which the formula applies.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that 'r' refers to the distance from the center of the massive object.
  • Others clarify that it is the distance between the centers of the two objects involved.
  • There is a mention that the formula applies under the assumption that the massive object is spherical, which is true for celestial bodies like the Earth, Sun, and Moon.
  • One participant questions whether 'r' should be squared in the formula, while another asserts that it should not, emphasizing that the formula is for gravitational potential energy, not gravitational force.

Areas of Agreement / Disagreement

Participants generally agree that 'r' refers to the distance from the center of the massive object, provided it is spherically symmetric. However, there is disagreement regarding the use of 'r' squared in the formula, which remains unresolved.

Contextual Notes

The discussion does not resolve the implications of non-spherical mass distributions or the specific conditions under which the GPE formula is applicable.

nathangrand
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Does the r in GPE=-GM/r refer to the distance an object is from the centre or surface of the object of big mass M?
 
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nathangrand said:
Does the r in GPE=-GM/r refer to the distance an object is from the centre or surface of the object of big mass M?
The center. (And that should be GPE = -GMm/r; you left off the mass of the object.)
 
Well to be more exact it's the distance between the centers of the two objects.
 
Its the distance from the centre of the big object as long as the big object is spherical. (Which luckily the earth, sun and moon roughly are).
 
BruceW said:
Its the distance from the centre of the big object as long as the big object is spherical. (Which luckily the earth, sun and moon roughly are).
Right. It requires a spherically symmetric mass distribution, which allows us to treat a large mass as a point mass.
 
Is it not r2 in the above formula?
 
JaredJames said:
Is it not r2 in the above formula?
No. It's for gravitational PE, not force.
 

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