The above simulation was created with EJS (all the simulations under category: [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php#3]Easy Java Simulations (2001- )[/url]  are the same)

The ejs source code is an xml file.
Assume you have install [url=http://fem.um.es/EjsWiki/index.php]Easy java simulation[/url]: You can open the source with EJS by

[b]Right click[/b] inside the simulation region, and select [b]open EJS model[/b] .

The perpendicular force is the force required for circular motion $\vec{F}=-\frac{v^2}{r}\hat{r}$