---
title: "Fundamental principles of applied mechanics; rigid body"
subject: "Applied Mechanics"
chapter: 1
key: "semester_1/applied_mechanics:fundamental-principles-define-rigid-body"
source: https://www.bctengineer.com/q/semester_1/applied_mechanics/fundamental-principles-define-rigid-body
site: BCT Engineering
---

# Fundamental principles of applied mechanics; rigid body

Applied Mechanics · Chapter 1: Introduction

- Asked in: 2073 Chaitra, 3+1 marks
- Video: [Lec-1 Intro to Engineering Mechanics, Fundamental Concepts, Units System](https://www.youtube.com/watch?v=lwOgbQO5HrQ) — Muhammad
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_1/applied_mechanics/fundamental-principles-define-rigid-body
- Whole chapter: https://www.bctengineer.com/md/semester_1/applied_mechanics/chapter-1

## Question

Enlist the fundamental principles of Applied Mechanics. Define rigid body.

## Short answer

The fundamental principles are Newton’s three laws of motion, the law of gravitation, the parallelogram law of forces, and the principle of transmissibility. A rigid body is an ideal body that does not deform; the distance between any two points remains constant under all forces.

## Answer

#### Fundamental principles of applied mechanics
1. **Newton's first law (law of inertia):** a body remains at rest or in uniform motion along a straight line unless acted on by an external force.
2. **Newton's second law:** the rate of change of momentum is proportional to the applied force and takes place in its direction, F = ma.
3. **Newton's third law:** action and reaction are equal in magnitude, opposite in direction and collinear.
4. **Newton's law of gravitation:** F = G m₁m₂/r², where G = 6.673 × 10⁻¹¹ N·m²/kg².
5. **Parallelogram law of forces:** the resultant of two concurrent forces is the diagonal of the parallelogram drawn with the forces as adjacent sides.
6. **Principle of transmissibility:** the external effect of a force on a rigid body is the same wherever the force is applied along its line of action.

#### Rigid body
A rigid body is an ideal body in which the distance between any two of its points remains constant under the action of forces, that is, it does not deform.

## Check yourself

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

1. Action and reaction between two bodies are:
   A. Unequal, with the larger one acting on the heavier body
   B. Equal in magnitude, opposite in direction and collinear
   C. Equal and opposite but acting along parallel lines
   D. Equal in direction but opposite in size
   Answer: B. Newton's third law states the pair are equal, opposite and collinear. (Key point: Newton's third law)

2. If the distance r between two masses is doubled, the gravitational force F = G m1m2/r^2 becomes:
   A. One eighth
   B. One quarter
   C. Double
   D. Half
   Answer: B. F is proportional to 1/r^2, so doubling r gives 1/4 of the force. (Key point: Law of gravitation)

3. The resultant of two concurrent forces is:
   A. The diagonal of the parallelogram drawn on them as adjacent sides
   B. Their difference in magnitude
   C. The sum of their magnitudes
   D. The longer of the two forces
   Answer: A. The diagonal of the parallelogram gives the resultant in magnitude and direction. (Key point: Parallelogram law)

4. A rigid body is an ideal body that:
   A. Does not deform, so the distance between its points is constant
   B. Changes shape under small loads
   C. Has no mass
   D. Moves only in a straight line
   Answer: A. No deformation means the distance between any two points remains constant. (Key point: Rigid body)

## Related in Applied Mechanics

- [Principles of mechanics; perfectly rigid assumption](https://www.bctengineer.com/question.md?key=semester_1%2Fapplied_mechanics%3Aprinciples-applied-mechanics-perfectly-rigid) — 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
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- [Rigid body and perfectly rigid assumption](https://www.bctengineer.com/question.md?key=semester_1%2Fapplied_mechanics%3Arigid-body-perfectly-rigid-assumption) — Applied Mechanics, chapter 1, asked 2×
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