In order to interact with the world, people must be able to predict how it will unfold in the future, and these predictions must be updated regularly in light of new information. Here we study how the mind updates these predictions over time. Participants were asked to make ongoing predictions about the destination of a simulated ball moving on a 2D bumper table. We modeled these decisions by assuming people simulate the world forward under uncertainty. This model fit participants’ behavior well overall, suggesting that people continuously update their physical simulations to inform their decisions. In some specific scenarios participants’ behavior is not fit well by the simulation based model in a manner suggesting that in certain cases people may be using qualitative, rather than simulation-based, physical reasoning.