.....on paper , as a common problem , at :
http://galileoandeinstein.physics.virginia.edu/lectures/gal_accn96.htm
.....as an "enlarged" test , at :
www.youtube.com/watch?v=FQ02MjqBk7s. (from 0:40 to 1:00 , only !)
.....so to "meet" a certain conditional , at :
www.peswiki.com/index.php/Directory:Gravity_Motors ...that is : "For a rotary device to *work* it has to achieve a full 360* rotation , with acceleration between each complete turn )
.....or better , as a natural suggestion , at :
www.youtube.com/watch?v=uGBANgbRkws
Al_ex
.....at : http://mechanicalexpressions.com/explore/dynamics/trebuchet.html
...if we asume that the length of the arm , from the fulcrum to the mass "m" , is oscillatory ("sliding" in some manner , to and fro along the arm ) so to play a continuous unbalance (torque difference) , on the same side of the fulcrum.
It's only a small "step of thinking"...
Al_ex