Computational thinking, an approach to problem solving, is a key practice of science education rarely integrated into instruction in an authentic way. A second key practice, creating models of physical phenomenon, has been recognized as an important strategy for facilitating students' deeper understandings of both science concepts and the practices of science. We are creating an interdisciplinary computational thinking curriculum for grades 4-6 that combines the development of computational thinking with content in other disciplines such as science. Here we present an example project where students can iteratively develop a model to explain the momentum and acceleration of an object, coupled with sophisticated computational thinking concepts to simulate that model. In addition, we present two findings from related research on fourth graders' pre-instructional knowledge related to computational thinking: 1) Students recognized the need for but struggled to produce specific instructions, and 2) Students understood that small errors could change outcomes.