title: Comment on "Mechanical analog of temperature for the description of force distribution in static granular packings".
abstract: It has been proposed by Ngan [Phys. Rev. E 68, 011301 (2003)] that the granular contact force distribution may be analytically derived by minimizing the analog of a thermodynamic free energy, in this case consisting of the total potential energy stored in the compressed contacts minus a particular form of entropy weighted by a parameter. The parameter is identified as a mechanical temperature. I argue that the particular form of entropy cannot be correct and as a result the proposed method produces increasingly errant results for increasing grain rigidity. This trend is evidenced in Ngan's published results and in other numerical simulations and experiments.
authors.affiliation: The KSC Applied Physics Laboratory, The John F Kennedy Space Center, NASA YA-C3-E, Kennedy Space Center, Florida 32899, USA. Philip.T.Metzger@nasa.gov
authors.forename: Philip T
authors.initials: PT
authors.lastname: Metzger