In this paper we investigate the dynamics of the process when a robot intercepts and captures a moving object. This operation is called dynamic mass capture. The effects of structural flexibility of the robot is taken into consideration. In terms of time the analysis is divided into three phases: before interception (finite motion), at the vicinity of interception and capture (impulsive motion), and after interception (finite motion). Special attention is paid to the modeling of the second phase when the robot intercepts and captures a target and it becomes part of the end effector, thus, the system’s degrees of freedom and topology suddenly change. To describe this event, an alternative approach is proposed. This is based on the use of a class of impulsive constraints, the so-called inert constraints. Jourdain’s principle is employed to derive the dynamic equations for both finite and impulsive motions. Based on the proposed approach, simulation results are presented for a flexible slewing link capturing a moving target. These results are compared with the observations of an experiment. Good agreement is found between the experimental and simulation results, which suggests that the analysis presented in this paper can be used with confidence in investigations of robots intercepting and capturing moving objects.