---
title: "Physical meaning of equilibrium and its application"
subject: "Applied Mechanics"
chapter: 2
key: "semester_1/applied_mechanics:physical-meaning-equilibrium-structural"
source: https://www.bctengineer.com/q/semester_1/applied_mechanics/physical-meaning-equilibrium-structural
site: BCT Engineering
---

# Physical meaning of equilibrium and its application

Applied Mechanics · Chapter 2: Basic Concept in Statics and Static Equilibrium

- Asked in: 2072 Kartik, 4 marks
- Video: [Engineering Mechanics | Theory of equilibrium | #11 | PCE | Prof. Sonali Parida](https://www.youtube.com/watch?v=vmDDZOrxMC0) — MES e-Learning
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_1/applied_mechanics/physical-meaning-equilibrium-structural
- Whole chapter: https://www.bctengineer.com/md/semester_1/applied_mechanics/chapter-2

## Question

Explain the physical meaning of equilibrium and its application in structural engineering.

## Short answer

Equilibrium physically means all forces and moments cancel, so a body has no tendency to translate or rotate. In structural engineering, it is used to find support reactions, internal forces (axial, shear, moment) via cuts, analyze trusses, check stability and determinacy, and design foundations against sliding and overturning.

## Answer

#### Physical meaning of equilibrium
A body is in equilibrium when the forces and moments acting on it are balanced, ΣF = 0 and ΣM = 0. Physically, the body has no tendency to start moving, to speed up or to rotate. It stays at rest (static equilibrium) or moves at constant velocity. In an equilibrium state, every external action on the body is matched by an equal and opposite reaction.

#### Application in structural engineering
- **Support reactions:** the equilibrium equations give the reactions at the supports of beams, frames, arches and trusses from the applied loads.
- **Internal forces:** cutting a member and applying equilibrium to one part gives the axial force, shear force and bending moment at that section, which are needed to size the member.
- **Truss analysis:** the method of joints (ΣFx = 0, ΣFy = 0 at each joint) and the method of sections give member forces.
- **Stability:** a structure that cannot satisfy equilibrium under a load would move or collapse, so equilibrium is the basic condition of a safe design.
- **Determinacy:** counting the unknowns against the equations of equilibrium shows whether a structure is statically determinate or indeterminate.
- **Foundations and retaining walls:** equilibrium of forces and moments is checked against sliding and overturning.

Thus all structural design starts with the requirement that the structure and each of its parts are in equilibrium.

## Check yourself

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

1. Physically, equilibrium means the body:
   A. Has no forces acting on it
   B. Is always at rest
   C. Is deforming steadily
   D. Has no tendency to start moving, speed up or rotate
   Answer: D. Forces and moments balance; the body is at rest or moves at constant velocity. (Key point: Meaning)

2. In structures, the equilibrium equations are used to find:
   A. The colour of the material
   B. The modulus of elasticity
   C. The cost of the structure
   D. The support reactions from the applied loads
   Answer: D. They give reactions of beams, frames, arches and trusses. (Key point: Reactions)

3. Cutting a member and applying equilibrium to one part gives:
   A. Axial force, shear force and bending moment at that section
   B. Only the weight
   C. Nothing useful
   D. The member's material
   Answer: A. These internal actions are needed to size the member. (Key point: Internal forces)

4. A structure that cannot satisfy equilibrium under a load will:
   A. Carry the load by friction
   B. Become statically determinate
   C. Become stronger
   D. Move or collapse
   Answer: D. Equilibrium is the basic condition of a safe design. (Key point: Stability)

## Related in Applied Mechanics

- [FBD and equilibrium: meaning and importance](https://www.bctengineer.com/question.md?key=semester_1%2Fapplied_mechanics%3Afbd-physical-meaning-equilibrium-importance) — Applied Mechanics, chapter 2
- [Meaning of equilibrium; drawing good FBD](https://www.bctengineer.com/question.md?key=semester_1%2Fapplied_mechanics%3Aequilibrium-meaning-importance-good-fbd) — Applied Mechanics, chapter 2

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