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: http://edu.kde.org/step