sentences:  The total force between the spheres is found to be
  where formula_167 are the radii of the two spheres.The two spheres separate completely when the "pull-off force" is achieved at formula_168 at which point
  
  To incorporate the effect of adhesion in Hertzian contact, Johnson, Kendall, and Roberts formulated the JKR theory of adhesive contact using a balance between the stored elastic energy and the loss in surface energy.The JKR model considers the effect of contact pressure and adhesion only inside the area of contact.