In a cyber-physical system (a system where the physical world interacts extensively with - often networked - software), the physical portion of the system resides in the continuous and continual domain. Thus, on the physical side of cyber-physical systems we will have to contend with not only real time requirements but also the continuous and continual nature of the system. This poses a new set of challenges for requirements engineering; we must write well defined requirements to address crucial issues not commonly addressed in the software domain. For example, the rate of change of a controlled variable, the time it takes for a controlled variable to settle sufficiently close to a set-point, and the cumulative errors built up over time may be of critical importance. In this paper we outline how early modeling in the continuous domain serves as a crucial aid in the elicitation and discovery of requirements for cyber-physical systems and provide an initial classification of the types of requirements needed to describe crucial aspects of the physical side of a cyber-physical system.