(Mass) moment of inertia is a concept applicable in the case of:
Try it first: pick an option, then read the explanation.
Correct answer: A. a rotating body
Asked in SSC JE 2009
Detailed Explanation
Mass resists change in linear motion (F = ma). Its rotational counterpart, mass moment of inertia I = Σmr², resists change in angular motion (T = Iα).
Why “a rotating body” Is Correct
The concept belongs to a rotating body. A body in pure translation needs only its mass. (In structures, the "area" moment of inertia used in bending is a geometric analogue of the same idea.)
| Linear motion | Rotational motion |
|---|---|
| Mass m | Moment of inertia I = Σ m r² |
| F = m a | T = I α |
| KE = ½ m v² | KE = ½ I ω² |
Why the Other Options Are Wrong
- B. a body moving in a straight line: Straight-line motion is governed by mass alone.
- C. a body at rest: A body at rest has no motion to resist in this sense (the area moment of inertia is a separate geometric property).
- D. both (a) and (b): Translation does not need the moment of inertia.
Key Point
Mass moment of inertia is the rotational analogue of mass.
Key Concepts for Students
- Flywheels store energy using large I.
- Area moment of inertia (mm⁴) is used for bending; mass MI is kg·m².
- Radius of gyration k: I = m k².
Practice more: this is question 40 in Bending and Shear Stress MCQs – Part 3. Try the whole set and check every answer instantly.