A body is said to be in equilibrium if:
Try it first: pick an option, then read the explanation.
Asked in SSC JE 02/03/2017 (Evening)
Detailed Explanation
A body is in equilibrium when the forces on it balance completely: there is no net force and no net moment. By Newton's first law it then stays at rest, or keeps moving with constant velocity in a straight line without rotating faster or slower.
\[ \Sigma F = 0 \quad\text{and}\quad \Sigma M = 0 \]
Why “None of these” Is Correct
The options describe motions, not the balance of forces. Moving horizontally or vertically, or rotating about the C.G., does not by itself tell us whether the forces balance — a body could be accelerating while doing any of these. So none of them is the condition of equilibrium: the answer is (d).
| Force system (in a plane) | Conditions of equilibrium |
|---|---|
| Concurrent forces | ΣH = 0, ΣV = 0 |
| Parallel forces | ΣF = 0 (along the forces), ΣM = 0 |
| General (non-concurrent) forces | ΣH = 0, ΣV = 0, ΣM = 0 about any point |
Why the Other Options Are Wrong
- A. it moves horizontally: A body moving horizontally may be accelerating; motion alone does not prove balance.
- B. it moves vertically: The same applies to vertical motion.
- C. it rotates about its C.G.: A body rotating with angular acceleration has an unbalanced moment.
Key Point
Equilibrium: ΣF = 0 and ΣM = 0 (rest or uniform motion).
Key Concepts for Students
- Static equilibrium: body at rest.
- Dynamic equilibrium (d'Alembert): add the inertia force −ma and treat as static.
- A free-body diagram shows every force acting on the isolated body.
Practice more: this is question 1 in General Conditions of Equilibrium MCQs – Part 1. Try the whole set and check every answer instantly.