Glad you asked. Well, turn out that the ground exerts what is called a "normal" force on the object, but that normal force is perpendicular to the ground. Since gravity always pulls straight towards the center, the normal and gravitational forces will be at an angle to each other if the ground is slanted. For a visual of this explanation take a look at the third diagram from the left below.
So what happens to the object. Well you can also see that there is a bit of friction force that works parallel to the ground (headed up and to the right in this case). Splitting the gravitational force into two parts, one parallel to the normal force (labeled F⊥) and one parallel to the frictional force (labeled F//) we now see that the normal and perpendicular cancel each other out so that the object doesn't fly off or sink into the ground; and the frictional and parallel forces cause the object to roll in the direction of greatest force, which will either be nowhere or down, depending on the amount of friction and how steep the surface is. The actual calculations are easy because you now have a right triangle, which means Pythagorean Theorem, which you may remember from your high school Geometry class.
Right about now you are starting to think that I tricked you. I pulled you into this post with promises of home restoration stories and instead I've made you read about math. Two points. One, that's what we math teachers do, and two, it's all about to come together.
I may have mentioned that I was planning on hanging the French doors between my living room and the front porch on rolling track hardware. The living room itself is fairly small and it has a double-wide entry in two of the walls and a regular door in a third wall, not leaving much in the way of actual usable wall space. Four chairs, three end tables, two lamps, and a TV fill the available space and, most importantly for the purposes of this story, don't allow the French doors to be opened or shut without moving furniture.
First step, build door trim around the French doors since they don't have any.
Now I hang rolling door track on the crosspiece, attach the brackets to the doors (I did have to drill holes in the 100 year old doors but in this case I think it is worth it and those can always be filled and painted down the road if necessary), and hang the doors.
Looks great, I couldn't be happier. So why all the fuss about vectors and gravity at the beginning of this post?
Well, here's what happens if I remove the clip that is keeping the doors from rolling freely.
That's right. My house is not level. When I built the door casement I built it to be level with the doors so that it wouldn't look off. When I hung the track I hung it to be level with the casement so that it wouldn't look off. This means that the casement, and therefore the door track, slant in a slightly downwards direction as one proceeds westerly. The laws of physics being what they are, and Ffriction being negligible on these tracks, F// is greater than Ffriction and the doors happily roll to the right until they hit the door stopper.
Not sure what the solution is in the long term, in the short term I have a binder clip on the left hand door to keep it from rolling closed and I move the binder clip when I want the doors to stay shut. For now I guess it will do but other than jacking up the corner of the house to level everything out, I am in the market for a better solution.
It still looks great, however, and I love the fact that I can now open and close the doors without moving furniture. So there's that.
Oh, and I still have to build the moulding for the top of the door casing. Trying to match the original which was either custom made or just isn't popular (and therefore available) in that exact design. Stand by for a future post.