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Motion of Rigid Body — Revision Notes

NIOS · Class 12 · Physics

Motion of Rigid Body revision notes for NIOS Class 12 Physics: 4 topics in quick points. Part of the NIOS Class 12 Physics syllabus.

45 questions26 flashcards17 formulas & key relations5 concepts

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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 Rigid Body – Definition and Concept, Centre of Mass (CM), Translational vs Rotational Motion, Moment of Inertia. Study these first, then practise questions on each for the NIOS Class 12 board exam.
How should I revise Motion of Rigid Body for the NIOS Class 12 board exam?
Learn the core ideas first, then work through the 45 practice questions on Motion of Rigid Body. Revise 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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