# "Tensegrities : structures made with cables and struts - Why do they hold up?" by Robert Connelly

Imagine a heavy object suspended by cables in midair. Imagine several objects suspended by cables yet so that the whole structure is completely stable. This is a tensegrity, a word suggesting "tensional integrity" by R. Buckminster Fuller after having been shown some models by the artist Kenneth Snelson in the 1940's. They seem to defy gravity, and yet examples of them are all around us as bridges, bicycle wheels, tents, spider webs, cloths lines, atoms, pebbles in a box, as well as works of art by Kenneth Snelson himself. What principles explain their stability? Where is the geometry? What is a good a geometric model? A very natural way to do this is to think of a tensegrity as a collection of points, where some pairs, the cables, are not allowed to get further apart, while others, the struts, are not allowed to get closer together. If these conditions determine the configuration of points up to rigid motions of the whole configuration, then the tensegrity is rigid. The secrete to this rigidity is energy. Indeed, you can even invent your own special seemingly unrealistic energy that explains the geometric rigidity, while in the end it is quite consistent with actual physical properties of the materials themselves. Geometry and Physics live together in harmony.