These bulldozers are motorized and travel at a constant velocity. When one bulldozer is placed on the desktop and another is placed on a moving cart or paper sheet you can see the relative velocity between the two moving carts. One dimensional and two dimensional relative velocities can be viewed. For those who would like to see this experiment but not actually perform it "live", I also have this experiment on video laserdisc.

Equipment
Constant Velocity Bulldozer