Step is an interactive physics simulator. It works like this: you place some bodies on the scene, add some forces such as gravity or springs, then click "Simulate" and Step shows you how your scene will evolve according to the laws of physics. You can change every property of bodies/forces in your experiment (even during simulation) and see how this will change the outcome of the experiment. With Step you can not only learn but feel how physics works.
WWW: https://projects.kde.org/step