Slow-Side Selection as a Unifying Foundation for Curling Stone Curl: A Conceptual Synthesis

The physical origin of the curl of curling stones has remained contested for nearly acentury. Two principal mechanisms have been proposed: a scratch-guide model in whichfront-edge asperities scratch the ice at a rotationally determined angle, guiding thetrailing edge laterally; and a discrete-pivot model in which the stone's center of massswings eccentrically around momentary high-friction grip contacts. These models aretypically presented as competing alternatives. This paper argues that both models share asingle physical foundation — slow-side selection — arising from the partial cancellationof translational and rotational velocity vectors on the slow side of the contact band,which reduces net sliding speed and increases local friction there. The two models differonly in their description of what each contact event does, not in where contactspreferentially form. Both models predict that curl diminishes at high rotational speedsrelative to translational speed, consistent with the empirically observed straightening of'spinner' shots in competitive play. A limiting case of near-zero translational velocity isexamined: in this regime the slow-side selection mechanism weakens, and a secondaryeffect — grip of the leading edge on untracked ice — may produce a small leftward drift.The lateral displacement of a nearly stationary, spinning curling stone does not appear tohave been systematically reported in the literature, and a simple experimental protocol toinvestigate this regime is proposed.