Wetting processes of droplets are omnipresent in both nature and technology. However, the behavior of a sliding droplet is far from being understood and there is a lack of knowledge in quantifying the forces acting on the droplet during motion. We have performed 3D Direct Numerical Simulations (DNS) of droplets wetting various substrates and evaluated the forces which act on them during motion. The simulations are performed using our diffuse-interface phase-field solver \texttt{phaseFieldFoam} implemented in OpenFOAM (FOAM-extend 5.0). The solver has been enhanced by load-balanced adaptive mesh refinement and a moving reference-frame technique, to refine the mesh at the interface and to follow the droplet’s center of mass, respectively. This substantially reduces computational costs. We specifically investigate the influence of capillary and viscous forces on the droplets' motion on substrates with varying surface chemistry. We also discuss the mechanisms of viscous dissipation as well as the viscous energy dissipation field within the droplet as it moves.