The idea of microworlds has been around since the 70s. These are typically small java based mathematical simulations of various principles. The main idea is they allow the user to play with the parameters of a simulation in order to understand how a system works. There are many of these that have been developed over the years on the internet, although they are becoming more unfashionable as some have moved onto smart phone development. Below are some links, but as usual before making a copy to distribute on the web or on DVD we ask you to check with the copyright holders. It should also be mentioned that you should check the quality of the particular microworld you want your students to use.