In physics, a coupling constant or gauge coupling parameter (or, more simply, a coupling), is a number that determines the strength of the force exerted in an interaction. Originally, the coupling constant related the force acting between two static bodies to the " charges " of the bodies (i.e. the electric charge for electrostatic and the mass for Newtonian gravity) divided by the distance squared, r 2 , between the bodies; thus: G in F = G m 1 m 2 / r 2 for Newtonian gravity and k e in F = k e q 1 q 2 / r 2 for electrostatic. This description remains valid in modern physics for linear theories with static bodies and massless force carriers.