This video illustrates a sphere rolling down a rail, with varying width.
Courtesy of the Wolfram Demonstrations Project
(Trouble? Download and install the Wolfram CDF player here. We use it a lot, and it does some really cool stuff.

Remember those two masses on an air track, connected with springs? This simulates two cylinders connected with springs.

Courtesy of the Wolfram Demonstrations Project
(Trouble? Download and install the Wolfram CDF player here. We use it a lot, and it does some really cool stuff.

A cool demo of a pulley with mass (aka Atwood Machine with pulley mass).
Courtesy of the Wolfram Demonstrations Project
(Trouble? Download and install the Wolfram CDF player here. We use it a lot, and it does some really cool stuff.

Just two simple gears to play with.

Courtesy of the Wolfram Demonstrations Project
(Trouble? Download and install the Wolfram CDF player here. We use it a lot, and it does some really cool stuff.