A system of particles is called a rigid body when the distance between any two particles of the system remains constant with time, no matter what forces or torques act on it.

Quick Scoop: Core Idea

In simple words, imagine you freeze the shape of an object forever:

  • The shape and size never change.
  • The relative positions of all particles stay fixed.
  • The body can translate (move from place to place) and rotate , but it does not deform (no stretching, bending, or squashing).

So, a system of particles is a rigid body when for every pair of particles iii and jjj, the distance between them is always the same, regardless of motion or external forces.

Why it’s an “ideal” model

In reality, no actual solid is perfectly rigid:

  • Under large forces, real solids deform slightly (like a rod bending or a bridge sagging).
  • In physics (especially at JEE / class 11–12 level), we idealize many solids as rigid bodies so that:
    • Rotational motion is easier to analyze.
    • We ignore internal deformation and focus on motion of the whole object.

Example:

  • A steel rod being rotated is treated as a rigid body in problems, even though, in reality, it vibrates and deforms a tiny bit under force.

Typical MCQ-style statement

If your question is from an exam/MCQ like:

A system of particles is called a rigid body when …?

Then the correct conceptual condition is:

  • “Any two particles of the system have zero relative velocity (or equivalently, the distance between them remains constant with time).”

That captures the same idea: they all move together like one solid piece. Information gathered from public forums or data available on the internet and portrayed here.