In this chapter, we start with the definitions of a link’s mass, a link’s center of gravity, and a link’sMass mass moment of inertia as well as the way of calculating them from the link's shapeShape, dimensions, and material. After that, we introduce how we useEngineering, newton Newton’sNewton second lawNewton’s second law in developing a dynamic modelDynamic model. The solution of this model could be developed as a forward dynamic model solutionDynamic model solution (calculating the forces and/or the momentsMoments from a known motion) or as an inverse dynamic model solution (calculating the motion from known forces and/or moments). Two full dynamic models are developed: the four-barFour-bar mechanismBar Mechanism dynamic model and the slider–crankSlider–crank mechanism dynamic model. Lastly, we present how to useSimscape Simscape™ Multibody™MultibodyTM to build a dynamic model-basedModel-based simulationSimulation of any machine and to calculate theActuatorsrequired required actuator forcesActuator forces andTorques torquesActuator forces/torques and reaction forces andMoments momentsForces and moments acting on a machine during a specific motion.