Kinematic and dynamic modeling of any physical process is critical especially for the design phase of associated control units and the prediction of their performance levels. In this paper, a dynamical model and a development frame including interactions with the objects for a tendon-driven underactuated hand are developed and examined. Using the proposed dynamic model, free movement and the object interaction of the underactuated hand can be simulated with a single, integrated simulation step in the simulation frame.Lagrangian Method is used to model the dynamic behavior of the underactuated fingers and the thumb under tendon dependency in a 3D coordinate system. Then, as an important extension to the modeling studies that can be found in the literature, the interaction of the hand with an object that co-exists in its workspace is modeled in terms of contact forces acting on the joints of the fingers. The unified dynamic model is simulated by using the Simulink platform.The simulations showed that the adaptability of the underactuated mechanism and joint torque levels are modeled and examined realistically. The flexion movement of the fingers resulted in realistic torque levels which can be handled by commercially-off-the-shelf actuators. Various tendon forces were examined in terms of the distance of the individual phalanx to the object and their contact instants. Applying an increasing input force with 1.617 N/s ramp slope, finger flexion movement was obtained in 1s. In another simulation scenario, compliance of the underactuated mechanism to an object was examined. With 0.66 N/s tendon force input ramp, proximal, middle and distal phalanx contacted an object in 1.3 s, 2.35 s and 2.75 s, respectively.Presented features obtained by the simulation platform are especially useful to researchers working on the development of control methods for underactuated prosthetic hands and robot manipulators. Through the simulations of experimental scenarios, a detailed analysis of dynamic responses of each finger phalanges can be carried out properly in any level of the design phase.