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Motion of Rigid Body

NIOS · Class 12 · Physics

Quick revision notes for Motion of Rigid Body — NIOS Class 12 Physics. Key concepts, formulas, and definitions for last-minute revision.

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A diagram illustrating the parallel axis theorem. It shows a rigid body with its center of mass, an axis passing through the center of mass (I_CM), and a parallel axis at a distance 'd' from the I_CM
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Key Topics to Revise

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1. Rigid Body – Definition and Concept

  • A rigid body is a system of particles where the separation between any two particles does NOT change during motion.
  • Shape and size of a rigid body remain preserved during its motion.
  • In reality, no perfectly rigid body exists — it is an idealisation. Solid objects like cricket balls, steel discs, and wooden blocks are good approximations.
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2. Centre of Mass (CM)

  • Centre of Mass (CM) is a mathematical point where the entire mass of the body can be assumed to be concentrated for the purpose of studying translational motion.
  • The CM may lie outside the physical body — for example, the CM of a ring lies at its geometric centre, which is empty space.
  • For a two-particle system with masses m₁ and m₂ at positions z₁ and z₂: z_cm = (m₁z₁ + m₂z₂)/(m₁ + m₂)
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3. Translational vs Rotational Motion

  • TRANSLATIONAL MOTION: All particles of the body move along parallel paths with equal displacements. The motion of a wooden block pushed across a floor is purely translational.
  • ROTATIONAL MOTION: All particles describe concentric circular paths about a fixed axis. Example: a grinding stone rotating about its central axis.
  • ROLLING MOTION: Combination of both translational and rotational motions. Example: a cylinder rolling on the floor.
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4. Moment of Inertia

  • Moment of Inertia (I) is the rotational analogue of mass. It measures the resistance of a body to changes in its rotational motion.
  • Definition: I = Σmᵢrᵢ², where rᵢ is the perpendicular distance of the iᵗʰ particle from the axis of rotation.
  • SI Unit of Moment of Inertia: kg·m²

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Full Notes

Key Concepts

A rigid body is a systemThe Centre of Mass isTranslational motion occurs when all particlesMoment of Inertia is the rotationalTwo theorems help calculate moment

Frequently Asked Questions

What are the important topics in Motion of Rigid Body for NIOS Class 12 Physics?
Key topics in Motion of Rigid Body include Motion of Rigid Body – Complete Chapter Overview, Mind map showing the major topics covered in the chapter on Motion of Rigid Body, Flowchart to identify whether a given object can be treated as a rigid body. These are the concepts NIOS Class 12 examiners draw on most — study them first, then practise related questions.
How to score full marks in Motion of Rigid Body — NIOS Class 12 Physics?
Understand the core concepts first, then work through the 45 practice questions available for this chapter. Revise formulas and definitions regularly, and use flashcards for quick recall before the exam.

Sources & Official References

Content is aligned to the official syllabus. Refer to the board website for the latest curriculum.

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