System rigidity What is the meaning of system rigidity in mechanics? I can't understand how to classify the system as rigid or not, and what is the effect of rigidity on the whole system.
If you know anything about system rigidity... Please let me know 
 A: It simply means that the object is so stiff that it can only move by proper Euclidean isometries. At any time, the whole body can only be a combination of rotations and translations of itself at any other time. In particular:
This means that the distance between every pair of points within the body stays the same at all times and all angles between lines in the body are invariant
Any combination of rotations and translations can be represented by a single translation followed by rotation (or rotation followed by translation - it doesn't matter as long as the order is consistent in a given discussion).
Can you represent the body at any time as a single translation followed by a rotation of itself at any other time? Can you translate to match up the centers of mass, then rotate into the correct orientation to find a perfect match? It is rigid if and only if the answer is yes.
So if a body is composite: e.g. a hinged system, or if it is squidgy like a jellyfish so that its surface deforms when pushed, then it does not move as described above and is not "rigid".
A: In the materials science context, rigid means hard, resisting deformation.  It is the opposite of elastic.  
If two material bodies are forced sideways against each other and one or both are elastic like rubber, it/they will deform, allowing them to squeeze past each other without much damage being done.
However, if both bodies are hard or rigid like stone, this will prevent them moving past each other.  They do not deform in response to the forces between them.  If there is enough force pushing them sideways they will grind against each other, causing the contact surfaces to wear down or break away until they can pass.  
If the forces push the bodies together, the deformation of elastic bodies increases the area of contact, which spreads out the force.  Hard surfaces deform very little, so the force remains concentrated at the contact point, causing rock or stone to fracture and break off.
