Young's equation describes the wetting phenomenon in terms of the contact angle between a liquid and a solid surface. However, the contact angle is not the only parameter that defines liquid-solid interactions, an additional parameter related to the adhesion between the liquid drop and the solid surface is also of importance in cases where liquid sliding is involved. It is postulated that wetting which is related to the contact angle, and interfacial adhesion, which is related to the sliding angle, are interdependent phenomena and have to be considered simultaneously. A variety of models that relate the sliding angle to the forces developed along the contact periphery between a liquid drop and a solid surface have been proposed in the literature. Here, a modified model is proposed that quantifies the drop-sliding phenomenon, based also on the interfacial adhesion that develops across the contact area of the liquid/solid interface. Consequently, an interfacial adhesion strength parameter can be defined depending on the mass of the drop, the contact angle and the sliding angle. To verify the proposed approach the adhesion strength parameter has been calculated, based on experimental results, for a number of polymer surfaces and has been correlated with their composition and structure. The interaction strength parameter can be calculated for any smooth surface from measurements of the contact and the sliding angles.