Notices
Results 1 to 7 of 7
Like Tree2Likes
  • 1 Post By Markus Hanke
  • 1 Post By KJW

Thread: Question : Increasing mass and the gravitational field

  1. #1 Question : Increasing mass and the gravitational field 
    Suspended
    Join Date
    Sep 2013
    Posts
    475
    Imagine this simpple case :

    A spherical mass with rather low density under a gravitational condition,
    resulting in a certain g factor at the surface and a dropping g-factor away from the sphere
    according to Newton's inverse square law.
    A gravitational field is formed of a certain strenght.

    Now imagine that i could gradually insert into the sphere an amount of extra mass,
    maintaining the radius and volume,
    increasing thus gradually the density and the mass of the objekt, and the strenght of the gravitational field.

    For instance mass x2 in the same volume > g at the surface x2, dropping in value further away.



    > What is fysically happening then during this event, to cause the gravitational field to get stronger ?

    We know the formulas for what is happening, we observe the resulting behaviour of the objekts,
    but how could we explain this transition as a process ?


    Reply With Quote  
     

  2.  
     

  3. #2  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    17,036
    What is fysically happening then during this event, to cause the gravitational field to get stronger ?
    You are increasing the mass that is present. This in turn increases the amount of space-time curvature. (Or, perhaps more accurately, the mass is the space-time curvature.)


    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  4. #3  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by Noa Drake View Post
    according to Newton's inverse square law.
    That's true only if the mass is small, has no angular momentum, and no net electric charge.

    but how could we explain this transition as a process ?
    Not sure what you are getting at. The physical process is that you are adding energy-momentum to the system, so the gravity changes as a result.
    Howard Roark likes this.
    Reply With Quote  
     

  5. #4  
    KJW
    KJW is online now
    Forum Professor
    Join Date
    Jun 2013
    Posts
    1,553
    Bear in mind that the extra mass has to come from somewhere, and that this mass always had a gravitational field associated with it. Then this gravitational field would simply combine with the original gravitational field to increase the total gravitational field.
    Howard Roark likes this.
    There are no paradoxes in relativity, just people's misunderstandings of it.
    Reply With Quote  
     

  6. #5  
    Suspended
    Join Date
    Sep 2013
    Posts
    475
    Quote Originally Posted by KJW View Post
    Bear in mind that the extra mass has to come from somewhere, and that this mass always had a gravitational field associated with it. Then this gravitational field would simply combine with the original gravitational field to increase the total gravitational field.

    Indeed a good remark.

    The intriguing thing there is, observing the force of 2 near masses, that 'combining' here is not 'adding' (summation of m1 and m2)
    but 'multiplication' (m1 x m2) leading to F.


    F = G x m1 x m2 / Rē

    Say

    m1 = 2

    m2 = 6

    > F ~ 12 (not an added 8)


    Obvously because the formula says so, and empyrically correct, but is it also obvious from observing the 2 gravitational fields ?


    > I would answer like this :

    m1 is present in a field that makes up the base state for the gravitational field of m1,
    and that field is produced by a mass m2, being 6, thus laying out the base state for m1 with factor 6
    Hence : 2 x 6 ~ 12 for F


    And similarly the other way :

    m2 is present in a field that makes up the base state for the gravitational field of m2,
    and that field is produced by a mass m1,being 2, thus laying out the base state for m2 with factor 2
    Hence : 6 x 2 ~ 12 for F


    Is that a correct way of looking at it ?
    Reply With Quote  
     

  7. #6  
    Moderator Moderator Janus's Avatar
    Join Date
    Jun 2007
    Posts
    2,223
    Quote Originally Posted by Noa Drake View Post


    Indeed a good remark.

    The intriguing thing there is, observing the force of 2 near masses, that 'combining' here is not 'adding' (summation of m1 and m2)
    but 'multiplication' (m1 x m2) leading to F.


    F = G x m1 x m2 / Rē

    Say

    m1 = 2

    m2 = 6

    > F ~ 12 (not an added 8)

    The formula you give is for finding the force acting between two masses at a given distance from each other, not for the force you would get from combining two masses. IOW, it is for the force at which they pull at each other.

    You multiply the masses because the gravity of each mass acts on the other. Mass 1 pulls on Mass 2 and vice versa.

    To use this formula to determine how the gravity field would change for an object as its mass changes, you set one of these masses to a set value, say 1, and then add to the other.

    Thus if you start out with a mass of 2, you get 2x1 = 2.

    If you increase the mass by 6, you get (2+6)x1 = 8

    It is additive.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


    Edit/Delete Message
    Reply With Quote  
     

  8. #7  
    Moderator Moderator Markus Hanke's Avatar
    Join Date
    Nov 2011
    Location
    Ireland
    Posts
    7,302
    Quote Originally Posted by Janus View Post
    It is additive.
    This is true in the Newtonian approximation, but not necessarily in the full GR formalism, especially not in the general case when angular momentum and electric charge are involved as well. The sources will combine in non-linear ways, which is one of the areas where GR differs from Newton.
    This just as a side note, for Noa Drake.
    Reply With Quote  
     

Similar Threads

  1. Light clock in a gravitational field
    By Howard Roark in forum Physics
    Replies: 26
    Last Post: July 26th, 2013, 11:31 PM
  2. Dark Matter, Dark Energy and the Hyperbolic Gravitational Field
    By Gary Anthony Kent in forum Astronomy & Cosmology
    Replies: 58
    Last Post: April 28th, 2013, 03:17 PM
  3. Replies: 74
    Last Post: April 27th, 2013, 11:45 AM
  4. Replies: 6
    Last Post: October 15th, 2010, 12:11 AM
  5. Unified Field Theory & Origin of Mass
    By Dr3adLoX in forum Personal Theories & Alternative Ideas
    Replies: 2
    Last Post: March 22nd, 2010, 11:19 AM
Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •