Cyber-Physical Systems (CPS) combine discrete and continuous physical processes. Developing a new cyber-physical system is an iterative process that involves design, simulation, prototyping, and production. Writing simulation code using current techniques is time consuming. My thesis is that a carefully designed, domain-specific language can alleviate this difficulty. To demonstrate this thesis, I designed and implemented a prototype of a novel simulation environment called Acumen. The centerpiece of Acumen is a physical description language called PhyDL, which allows the user to directly describe dynamic equations governing the system being modeled, making the writing of simulation code much easier for engineers. A series of automatic transformations convert the high-level descriptions into a form that is directly machine executable. This thesis presents the design of PhyDL and several case studies of using PhyDL for designing CPS systems, which show that PhyDL is accurate and easy to use for domain experts.