a system of particles is called a rigid body when

A system of particles is called a rigid body when the distance between any two particles of the system remains constant over time, regardless of external forces applied on it. This means there is no relative motion or deformation between particles; any two particles in the system have zero relative velocity. In other words, a rigid body maintains its shape and size under the action of forces, with no internal deformation occurring. This concept simplifies the study of motion and forces in physics since the body behaves as if it were perfectly solid and undeformable.