Using the Phys X SDK along with Direct X, we created a ground mesh in Maya along with an untextured bounding box to avoid the car disappearing off the edge of the terrain. We then added boxes and a car, with their own independent physics properties. The user could then move the car around to collide with the boxes, allowing Phys X to perform the calculations. Not all the boxes have the same weight, so some will move more freely when hit by the car while others will remain essentially still.
You can view a video of it in action here.