In physics education, virtual simulations have given us the ability to show and explain phenomena that are otherwise invisible to the naked eye. However, experiments with analog devices still play an important role. They allow us to verify theories and discover ideas through experiments that are not constrained by software. What if we could combine the best of both worlds? We achieve that by building our applications on a projected augmented reality system. By projecting onto physical objects, we can paint the phenomena that are invisible. With our system, we have built "physical playgrounds": simulations that are projected onto the physical world and that respond to detected objects in the space. Thus, we can draw virtual field lines on real magnets, track and provide history on the location of a pendulum, or even build circuits with both physical and virtual components.