It was years ago, but we did something similar in a physics class with a bucket with some sand in it and a couple of small boards. There are two key elements to make it work and look "impressive". The center of balance of the whole system has to be directly under the edge of the board, and the weight of the support part of the system is very small, relative to the weight of the object you're hanging. When you solve all the force vectors, it ends up "looking" like a plumb bob, and have you ever seen one of those that didn't work?