Multimodal language models often need to reason about the physical dynamics of the world in order to answer questions appropriately. Various methods have emerged for solving physical reasoning tasks, and in particular, two leading approaches are Chain-of-Thought reasoning and physics-grounded simulation. However, relatively little is known about which particular problems can be solved by each approach. Some tasks may benefit from computationally expensive generative video simulations, whereas others may be solved equally well (or even better) with language-based reasoning. Our work addresses this by systematically comparing simulation and language reasoning across a variety of multimodal tasks involving physical reasoning, such as answering questions about fluid dynamics and elasticity. We draw a parallel with this to resource-rational theories of human cognition, which propose that people optimally trade off simulation and reasoning depending on task demands. We present a novel cognitively-inspired hybrid model which optimally trades off simulation and reasoning to improve model performance and computational efficiency. Our model is able to decide the appropriate method to solve a physical reasoning problem at test time, and finds an optimal balance between accuracy and cost. Further, our model can be trained on a smaller vision-language model that is only 4B parameters and deployed to improve performance on much bigger base models with $>$3T parameters like Gemini 3 Pro.

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