---
title: "Concept of particle, rigid and deformable body"
subject: "Applied Mechanics"
chapter: 1
key: "semester_1/applied_mechanics:concept-particle-rigid-deformable"
source: https://www.bctengineer.com/q/semester_1/applied_mechanics/concept-particle-rigid-deformable
site: BCT Engineering
---

# Concept of particle, rigid and deformable body

Applied Mechanics · Chapter 1: Introduction

- Asked in: 2069 Chaitra, 3 marks
- Video: [Concept of Rigid Body and Deformable Body](https://www.youtube.com/watch?v=4UVygVqwWRk) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_1/applied_mechanics/concept-particle-rigid-deformable
- Whole chapter: https://www.bctengineer.com/md/semester_1/applied_mechanics/chapter-1

## Question

Describe briefly the concept of particle, rigid body and deformable body.

## Short answer

A particle is a body of negligible size with mass concentrated at a point, where only force equilibrium is needed. A rigid body maintains constant distance between points, ignoring deformation, requiring both force and moment equations. A deformable body undergoes shape and size changes, and is analyzed in strength of materials.

## Answer

#### 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. A body can be treated as a particle when:
   A. It does not move
   B. Its size is negligible, so its mass is concentrated at a point
   C. It is made of a rigid material
   D. It has no weight
   Answer: B. A particle has negligible size and all its mass at one point. (Key point: Particle)

2. For a particle, which equations are needed?
   A. Force and moment equations both
   B. No equations
   C. Moment equations only
   D. Force equations only
   Answer: D. Forces on a particle are concurrent, so only the force equations are needed; a rigid body needs moment equations too. (Key point: Equations needed)

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