Gravitational Potential -- Clarification needed while defining this term

Discussion in 'Physics' started by logearav, Apr 24, 2012.

  1. logearav

    Thread Starter Member

    Aug 19, 2011
    248
    0
    Dear Revered Members,
    The definition for Gravitational potential at a point is " The work done in bringing the unit mass from infinity to that point". My doubt is " Since force of gravity is attractive, why should we do some some external work in bringing the body from infinity to the said point". Won't it not the body get attracted towards the surface of the earth by virtue of attractive force?
    If we have to move the body from a point near the surface of the earth to some infinite distance, we have to do some external work against the attractive force of gravity.
    Can i get some clarification in this regard?
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,135
    1,786
    The thing about gravitational potential energy is that you can locate the zero point anywhere. In most problems in mechanics the surface of the earth is a good place for a zero point. Now the only variable in the computation is the height above the ground. I never heard the bit about infinity, but it sounds more relevant to the electric field potential. For example how much work is required to bring a point charge from infinity to a point near a collection of charges. Potential energy is all relative.
     
  3. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    why should we do some some external work in bringing the body from infinity to the said point". ... the work is negative.

    Infinity is often a good point for the zero of energy because then bound particles have negative total energy and unbound ones total energy greater than 0.
     
  4. MvGulik

    Member

    Nov 3, 2011
    40
    9
    Would that not be the energy equivalent to the escape velocity at that point.
    (only true zero (theoretical) at a infinite point away.)
     
  5. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    Not precisely stated, but basically yes.
     
  6. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    It's also important to qualify that it is the work needed to bring the particle from rest at infinity to rest at the point in question. This may, in turn, imply/require that we are dealing with an inertial reference frame.
     
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