---
title: "Compare static equilibrium and equilibrium"
subject: "Applied Mechanics"
chapter: 2
key: "semester_1/applied_mechanics:static-equilibrium-vs-equilibrium"
source: https://www.bctengineer.com/q/semester_1/applied_mechanics/static-equilibrium-vs-equilibrium
site: BCT Engineering
---

# Compare static equilibrium and equilibrium

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

- Asked in: 2067 Asoj, 3 marks
- Video: [Static Equilibrium and Dynamic Equilibrium | Physics | #equilibrium](https://www.youtube.com/watch?v=LBRtSuLnaE4) — Najam Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_1/applied_mechanics/static-equilibrium-vs-equilibrium
- Whole chapter: https://www.bctengineer.com/md/semester_1/applied_mechanics/chapter-2

## Question

Compare between the static equilibrium and equilibrium.

## Short answer

Equilibrium means resultant force and moment are zero, so acceleration is zero; velocity may be zero or constant. Static equilibrium is the special case where the body is also at rest, so velocity is zero. Every static equilibrium is equilibrium, but not vice versa.

## Answer

**Equilibrium** is the condition in which the resultant force and resultant moment on a body are zero (ΣF = 0, ΣM = 0), so the body has no acceleration. The body may be at rest or moving with constant velocity.

**Static equilibrium** is the special case in which the body is also at rest, so its velocity is zero as well as its acceleration.

| Point | Static equilibrium | Equilibrium (general) |
|---|---|---|
| Velocity | Zero, body at rest | Zero or constant |
| Net force and moment | Zero | Zero |
| Acceleration | Zero | Zero |
| Studied in | Statics | Statics and dynamics (uniform motion) |
| Example | A beam carrying a load, a bridge | A car moving at constant speed on a straight road; a body at rest |

Every static equilibrium is an equilibrium, but not every equilibrium is static. The same equations ΣF = 0 and ΣM = 0 are used for both.

## Check yourself

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

1. Which pair of conditions defines equilibrium of a body?
   A. Resultant force and resultant moment are both zero
   B. Resultant force is zero but moment may be nonzero
   C. Velocity is zero and acceleration is non-zero
   D. Resultant moment is zero but force may be nonzero
   Answer: A. Equilibrium means SigmaF = 0 and SigmaM = 0, so there is no acceleration. (Key point: Condition for equilibrium)

2. What extra condition makes equilibrium 'static'?
   A. The body is at rest (velocity is zero)
   B. The body is accelerating uniformly
   C. The body carries no load
   D. Only the force equation is satisfied
   Answer: A. Static equilibrium is the special case of equilibrium in which the body is at rest, so velocity as well as acceleration is zero. (Key point: Static case)

3. Which is an equilibrium but NOT a static equilibrium?
   A. A car moving at constant speed on a straight road
   B. A beam carrying a load
   C. A bridge under its own weight
   D. A book resting on a table
   Answer: A. Constant velocity means zero acceleration and zero net force, so it is in equilibrium, but the body is moving so it is not static. (Key point: Example)

## Related in Applied Mechanics

- [Equilibrium and its equations in 2D and 3D](https://www.bctengineer.com/question.md?key=semester_1%2Fapplied_mechanics%3Aequilibrium-equations-2d-3d) — Applied Mechanics, chapter 2

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