Celebrating 32 years experience working as a professional photographer
I do understand Newtonian physics and I agree with Alan: If you are only going to sandbag one leg, and the big leg is where it should be ( under where the mass the arm is supporting which isn't always directly at the end of the arm), sandbag the second leg. If the sandbag is on the side where the mass is, it cannot counteract the downward force of what ever you have attached to the end of the arm. and if you are worried about the sandbag dragging on the floor lay both sandbags across the leg instead of parallel to it.
Another (minor) correction: They are called Century stands because Century is the company that first invented them.
To protect the unwary who might be walking around the edges of a set, once the stand is set up put a bright colored tennis ball on the end of the arm that poking out behind the stand. You rather have someone bump their face on a tennis ball than a 5/8" metal rod.