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

# Fundamental concepts and principles of mechanics

Applied Mechanics · Chapter 1: Introduction

- Asked in: 2074 Chaitra, 3 marks; 2072 Chaitra, 3 marks; 2068 Shrawan, 4 marks; 2067 Asoj, 3 marks (asked 4 times)
- 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/mechanics-fundamental-concepts-principles
- Whole chapter: https://www.bctengineer.com/md/semester_1/applied_mechanics/chapter-1

## Question

Explain in brief about fundamental concepts and principles of mechanics.

## Short answer

The fundamental concepts are space, time, mass, force, particle, and rigid body. The principles are Newton’s three laws, the law of gravitation, the parallelogram law, and the principle of transmissibility.

## Answer

Applied mechanics is the branch of science that deals with the effect of forces on bodies at rest or in motion. It is built on a few basic concepts and a few basic principles.

#### Fundamental concepts
- **Space:** the region occupied by bodies. Position is fixed by linear and angular measurements from a reference frame.
- **Time:** the measure of the succession of events. It is important in dynamics.
- **Mass:** the quantity of matter in a body. It is a measure of inertia, the resistance to change of motion.
- **Force:** the action of one body on another that tends to change its state of rest or motion. It is a vector with magnitude, direction and point of application.
- **Particle:** a body whose size is so small that it can be treated as a point with mass.
- **Rigid body:** a body in which the distance between any two points stays constant under load.

#### Fundamental principles
1. **Newton's first law:** a body stays at rest or in uniform straight-line motion unless a net external force acts on it.
2. **Newton's second law:** the acceleration of a body is proportional to the net force and acts in its direction, F = ma.
3. **Newton's third law:** to every action there is an equal and opposite reaction.
4. **Newton's law of gravitation:** two bodies attract each other with a force F = G m₁m₂/r².
5. **Parallelogram law:** two forces acting at a point can be replaced by one resultant, the diagonal of the parallelogram formed on them.
6. **Principle of transmissibility:** the effect of a force on a rigid body does not change if the force is moved along its line of action.

## Check yourself

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

1. In mechanics, mass is best described as:
   A. The quantity of matter in a body, a measure of its inertia
   B. The pull of gravity on the body
   C. The region of space the body occupies
   D. The action of one body on another
   Answer: A. Mass is the quantity of matter and measures inertia, the resistance to a change of motion. (Key point: Mass)

2. Which statement defines a rigid body?
   A. A body with no mass
   B. A body small enough to be treated as a point
   C. A body that stretches in proportion to the load
   D. Distance between any two points stays constant under load
   Answer: D. A rigid body keeps the distance between any two of its points constant under load. (Key point: Rigid body)

3. According to Newton's second law, the acceleration of a body:
   A. Is inversely proportional to the net force
   B. Is zero whenever any force acts
   C. Is proportional to the net force and acts in its direction
   D. Is equal to the force and opposite to it
   Answer: C. F = ma: acceleration is proportional to the net force and in the same direction. (Key point: Newton's second law)

4. The principle of transmissibility says that for a rigid body:
   A. Action and reaction are equal and opposite
   B. Two forces can be replaced by one diagonal
   C. A force can be moved to any parallel line without change
   D. The effect of a force is unchanged if it is moved along its line of action
   Answer: D. Sliding a force along its own line of action does not change its effect on a rigid body. (Key point: Transmissibility)

## Related in Applied Mechanics

- [Fundamental principles of applied mechanics; rigid body](https://www.bctengineer.com/question.md?key=semester_1%2Fapplied_mechanics%3Afundamental-principles-define-rigid-body) — Applied Mechanics, chapter 1
- [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

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