This paper introduces a class of real-time systems denoted as Real-Time Physical Systems (RTPS), in which a physical quantity of interest is associated with a real-time resource. The physical quantity behavior is determined by scheduling events generated by a real-time scheduling algorithm. RTPS systems aim to generalize some existing models used in real-time computing systems, namely power-aware and temperature-aware systems. Moreover, they have been conceived to put a bridge across real-time systems and the rapidly growing research field of Cyber-Physical Systems. In this paper we focus on a specific physical system where a state variable changes with an exponential decay behavior and the associated real-time resource must be scheduled in order to bound the value of the state variable within the desired working range. The aim is to determine the relationship between those physical and real-time parameters. For this purpose interesting properties are highlighted and relevant results are derived regarding the considered system model.