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

# Concept of rigid and deformable bodies

Applied Mechanics · Chapter 1: Introduction

- Asked in: 2069 Asar, 4 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-rigid-deformable-bodies
- Whole chapter: https://www.bctengineer.com/md/semester_1/applied_mechanics/chapter-1

## Question

Write down the concept of rigid bodies and deformable bodies.

## Short answer

A rigid body is an idealization that does not deform; the distance between any two points remains constant, used in statics where transmissibility applies. A deformable body changes shape and size under load, with varying point distances, and is studied in strength of materials for stress and strain analysis.

## Answer

#### Rigid body
A rigid body is an ideal body that does not change its shape or size under any load. The distance between any two of its points stays constant. Statics assumes bodies to be rigid, so the equations of equilibrium can be applied to the original geometry, and the principle of transmissibility holds. A steel beam under a small load is treated as a rigid body when its support reactions are found.

#### Deformable body
A deformable (real) body changes its shape and size, that is, it stretches, bends or twists, when forces act on it. The deformation depends on the material and on the size of the load. The distance between its points changes. Deformable bodies are studied in strength of materials, where stress, strain and deflection are found. Rubber and a spring are good examples of bodies whose deformation is large.

| Point | Rigid body | Deformable body |
|---|---|---|
| Deformation | None | Present |
| Distance between points | Constant | Changes |
| Used in | Statics, dynamics | Strength of materials |
| Transmissibility | Valid | Not valid |

In practice every body is deformable, but when the deformation is very small it is neglected and the body is treated as rigid.

## Check yourself

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

1. Statics treats bodies as rigid so that:
   A. Equilibrium equations can use the original geometry
   B. Deformation can be measured
   C. Stress and strain can be found
   D. The material need not exist
   Answer: A. Rigid assumption lets equations use the undeformed geometry. (Key point: Rigid body)

2. A deformable body is one in which:
   A. Only mass matters
   B. Shape and size change under load
   C. The distance between points stays constant
   D. Forces do not act
   Answer: B. Deformable bodies stretch, bend or twist, so distances change. (Key point: Deformable body)

3. Stress, strain and deflection of deformable bodies are studied in:
   A. Statics
   B. Engineering drawing
   C. Kinematics
   D. Strength of materials
   Answer: D. Strength of materials treats the body as deformable. (Key point: Where deformable studied)

4. A real body may be treated as rigid when:
   A. It is very heavy
   B. It is made of rubber
   C. It is moving at constant speed
   D. Its deformation is very small and can be neglected
   Answer: D. Every body deforms, but very small deformation is neglected. (Key point: When rigid is acceptable)

## Related in Applied Mechanics

- [Rigid vs deformable bodies; free body diagram](https://www.bctengineer.com/question.md?key=semester_1%2Fapplied_mechanics%3Arigid-deformable-bodies-fbd) — Applied Mechanics, chapter 2, asked 2×
- [Concept of particle, rigid and deformable body](https://www.bctengineer.com/question.md?key=semester_1%2Fapplied_mechanics%3Aconcept-particle-rigid-deformable) — Applied Mechanics, chapter 1
- [Rigid and deformable body; FBD principles](https://www.bctengineer.com/question.md?key=semester_1%2Fapplied_mechanics%3Arigid-deformable-fbd-principles-static-equilibrium) — Applied Mechanics, chapter 2