currently we use closed software for this (crocodile clips / crocodile physics). But maybe someone knows good free software.

looks like there will be a strong free mechanical interactive software soon (free use in schools, but still closed source). it's working fine. i am just not 100% sure how to use it in school best, because i think our pupils can and should do that always in real practice.
have a look at:
http://www.phunland.com/wiki/Home

Looks like crocodile clips is now free for home use (NOT for school). (compare http://www.crocodile-clips.com/ )
but i am still searching for an alternative software.