Determines the overall contact model in this domain, i.e. are objects solid or fluid, etc. The elasticity is a constant which must be set that determines how much energy is lost in the collision. A value of 1.0 for elasticity means no energy is lost so the total energy after the collision remains the same even though it may have been transferred between the colliding objects. A value of 0.0 for elasticity means all the energy of the collision has been converted to heat so the object remain together. Although a value of 0.0 is not supported in the current implementation, values approaching 0.0 are supported.
Contact forces are the forces that prevent solid objects in contact from overlapping.
We model the forces parallel to the direction of contact, which allows objects to push against one another, or for one blob to push an inaminate blob, etc. The effects of friction on the blobs, the individual masses and intertia of the blobs, etc. are all considered. We don't model the forces perpendicular to the collision. We simply ignore them which is equivalent to assuming infininately strong friction forces between the object surfaces. See physical-contact for most of the implementation.