Georgian Mark DiBois asks:
One weird thing... I still can't find a small toy bulldozer to pull a mass across the table... Where do you get those?
Simple answer: In the science supply houses you're looking for what's called a "constant speed vehicle." Fool around on google and you can see the device pictured above, available from Frey, Fisher, and others for $25-30 each. I *think* these can move forward or backward at the flip of a switch.
Better answer: PASCO offers two levels of constant speed items. In a rare event, PASCO undercuts the competition with their "constant speed buggy", pictured here. These are $8 each. They only move one direction, but they work just as well as the more expensive bulldozers.
Coolest answer: If you have a few bucks lying around for luxury purchases, try out the PASCO "variable speed motorized cart" pictured here. It lists at $159. But it has a knob for adjusting speed, and has traction enough to climb reasonably steep ramps.
My personal solution: I have two of the $25 bulldozers and ten of the $8 buggies. (I like to have some of each because they move at different speeds.) I use these for laboratory investigations. I also have one of the fancy-pants variable speed carts, which I use exclusively for demonstrations, or even research problems where necessary.