Concept of particle, rigid and deformable body

Particle

A particle is a body of negligible size, so that its whole mass can be taken as concentrated at a single point. All forces on it are concurrent, so only the force equations are needed. A stone thrown through the air, or a car when its motion over a long distance is studied, can be treated as a particle.

Rigid body

A rigid body is a body in which the distance between any two points stays fixed, however large the loads. It is an ideal concept in which deformation is ignored. The size and shape of the body matter, so both force and moment equations are needed. A beam or a wheel is usually treated as a rigid body in statics.

Deformable body

A deformable body changes its shape and size under load. The deformation depends on the material and the load. It is the true behaviour of real bodies and is studied in strength of materials, where stresses, strains and deflections are calculated. A rubber band or a loaded bridge member is an example.