Algodoo

Looks interesting, but I cannot figure out how to use it to make gears. Could you provide some instructions?

This was made to show a concept and not as a tutorial but here's the general steps.
1. Make a rack.
2. Make a circle that has a perimeter equal to an integral number of gear teeth.
3. Make a device to roll the rack pitch line about the circle perimeter.
4. Make clones of the rack as you roll it around the perimeter.
5. Delete everything except the cloned racks.
6. Merge all the racks together.
7. Trim off the excess to get what looks like an internal gear.
8. Use "internal gear" as a cookie cutter.
9. Add a central hinge.