Class 11 Physics Chapter 6 System of Particles and Rotational Motion – Revision Notes

Rotational motion in Chapter 6 of Class 11 Physics is built around torque, moment of inertia, and angular momentum, the rotational counterparts to force, mass, and linear momentum. It also gives the moment of inertia formulas for common shapes and explains how angular momentum stays constant when no external torque acts.

Last Updated: September 23, 2026

Key Concepts

• Centre of mass: X_cm=Σmx/Σm.
• Torque: τ=r×F.
• Angular momentum: L=Iω; τ=dL/dt.

Moment of Inertia

• Sphere: (2/5)MR²; Cylinder: (1/2)MR²; Rod (centre): (1/12)ML².
• Parallel axis: I=I_cm+Md². Perpendicular axis: I_z=I_x+I_y.

Conservation and Rolling

• Angular momentum conserved when net torque=0.
• Rolling KE=½Mv²+½Iω².

One-Line Summary

Rotational motion is described using torque, angular momentum, and moment of inertia, with angular momentum conserved in the absence of external torque, analogous to linear momentum conservation.

Quick visual: a worked diagram from the full Solutions page, for reference.

Frequently Asked Questions

What is the centre of mass of a system of particles?
The centre of mass is a single point that represents the average position of all the mass in a system, weighted by the mass of each particle. It moves as if the entire mass of the system were concentrated there.

How is torque different from force in rotational motion?
Force causes linear acceleration, while torque, which depends on the force and the perpendicular distance from the axis of rotation, causes angular acceleration, playing the same role in rotational motion that force plays in linear motion.

Written by Satish

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