JEE Main Physics Angular Momentum and Rigid Body Dynamics Handwritten Notes

JEE Main Angular Momentum Handwritten Notes, free to download as a 20-page PDF. The topic is worth about 4 to 8 marks and takes roughly one day to learn, so treat it as a single focused sitting.

The notes are scans of a real notebook, so they read the way a topper's own pages do: short lines, boxed formulas, and a margin note where something is easy to get wrong.

JEE Main Angular Momentum Weightage

This is the second half of Unit 5. Conservation of angular momentum is the part NTA names applications for, so expect it as a reasoning question as often as a calculation.

What the Notes Cover

• Angular momentum of a particle and of a rigid body
• Torque as the rate of change of angular momentum
• Conservation of angular momentum, and what happens to the kinetic energy
• The applications NTA names: skaters, divers, planets, collapsing stars
• The three equations of rotational motion for constant angular acceleration
• A full comparison table of linear and rotational quantities
• Equilibrium of rigid bodies, and how to choose a pivot
• Toppling, and when a body slides instead

How to Approach the Chapter

Almost every question here is one of four kinds, and naming which one before writing anything saves more time than any shortcut.

A shape change, or something landing on a spinning body, means conserve angular momentum. Constant torque with a time asked for means the equations of motion. Two states with no time mentioned means energy. Nothing moving at all means the two equilibrium conditions.

For equilibrium, remember that the torque sum is zero about every point, so you may choose the pivot. Pick one where an unknown force acts and that unknown drops out of the equation before you start.

Watch for the case where the moment of inertia itself changes. Torque equals I alpha does not survive that; only torque equals dL by dt does.

Common Mistakes

• Conserving kinetic energy when something sticks to a spinning body. Conserve L
• Using torque equals I alpha while I is changing
• Leaving omega in revolutions per minute inside an equation of motion
• Checking only forces for equilibrium and forgetting the torque condition
• Forgetting the weight of the beam itself, which acts at its centre if uniform
• Thinking a particle moving in a straight line has no angular momentum

Video Revision

Source: JEE Wallah

How to Use These Notes

Page 8 is the comparison table between linear and rotational motion. If that table is secure, most of the rest of this chapter is something you already know written in different symbols.

Page 15 is the which-law-when decision list, and it is worth reading before every practice set until the choice becomes automatic.

Torque, moment of inertia and the axis theorems are covered in the first set of Unit 5 notes.