There is a site with interactive physics apps you might like, and these apps work in an iPad as well as real computers. The site is www.geogebratube.org. Here is one example.
If you cannot see the interactive app above, go directly to the source here.
These apps are made with the FOSS program Geogebra which can be downloaded at www.geogebra.org. Geogebra is not that hard to learn to use and the apps it makes can be used on any computer which has Java. I have made a couple and placed them on my class webpage. You can even download source files from geogebratube and modify them if you want.
While the following apps are not directly on Geogebratube, I recommend that you check out the ones made by Daniel Mentrard at http://dmentrard.free.fr/GEOGEBRA/index.htm. He has a large collection covering all areas of what we teach. Check out this from his site.
I have found that if you erase htm (or html) in the address bar of your browser and replace it with ggb, you can usually download the source file so that you can replace any French text with English before using it in class or you modify it in any other way.
Currently the only way that a student can interact with these apps on an iPad is through the Geogebratube site, but there are indications that future developments in the Geogebra program will allow you to embed these apps into an ebook that can be read on an iPad directly without going through a web-browser.
While most of the apps you find at Geogebratube are not as refined or sophisticated as ones that you may already be using from PhET (phet.colorado.org) or Open Source Physics (www.opensourcephysics.org), you can make these apps without paying for program to make Flash apps and your students using iPads will not be left out. Since my school is going to be an "iPad school", this tool looks like it will be a major one used in my classes in the future.