Deformable object modeling has been widely used in computer graphics rendering and haptic rendering. We observe that many deformable objects in our daily lives are not totally arbitrary and preserve some geometric properties under deformation related to physical properties. In this paper we derive from physical properties some geometric properties of contacts between a rigid object and an elastic object that govern the shape change of the elastic object under deformation. Both local and global deformation are considered. These properties could be useful for more physically accurate and faster simulation and collision detection involving such deformable objects in many applications.