Rotational Motion – Page 9

How to Tackle ANY Objective Problem (IIT-JEE Level)

This page is a thinking manual. If you apply these steps, even unknown questions become solvable.

STEP–1: Identify the Nature of Motion

Ask immediately:

• Pure rotation?
• Pure translation?
• Rolling (translation + rotation)?
• Sudden interaction (collision, sticking, breaking)?

Tip: 70% mistakes happen by misidentifying motion.

STEP–2: Decide the RIGHT Conserved Quantity

Ask:

• External torque = 0 → conserve angular momentum
• No non-conservative work → conserve energy
• Sudden events → angular momentum (not energy)

Golden Rule: Never conserve energy in collisions.

STEP–3: Fix Reference Axis (VERY IMPORTANT)

Angular momentum depends on axis.

Choose axis:

✔ Through hinge
✔ Through centre of mass
✔ Where external torque = 0

Examiner Trap: Wrong axis ⇒ correct formula gives wrong answer.

STEP–4: Use I / MR² Ratio for Speed

For rolling problems:

a = g sinθ / (1 + I/MR²)

Remember quickly:

Ring → 1
Disc → 1/2
Sphere → 2/5
Rod (centre) → 1/12

Shortcut: Smaller I/MR² → faster motion.

STEP–5: Check Role of Friction

Never assume friction always does negative work.

Cases:

• Pure rolling → work by friction = 0
• Sliding → friction dissipates energy
• Starting rolling → friction increases ω

Topper Insight: Friction can increase rotational KE.

STEP–6: Energy vs Angular Momentum Decision

Use this rule:

✔ Sudden event → Angular momentum
✔ Smooth evolution → Energy
✔ External torque present → Energy only

STEP–7: Eliminate Options Before Calculation

Objective questions reward elimination.

Check:

• Dimensions
• Limiting cases
• Physical sense (zero, infinity)

Tip: Many IIT questions are solvable without full math.

COMMON IIT EXAM TRAPS (ROTATIONAL)

❌ Conserving wrong quantity
❌ Ignoring reference axis
❌ Assuming rolling without statement
❌ Mixing linear & angular formulas
❌ Forgetting rotational KE

TOPPER MINDSET (WHAT DIFFERENTIATES THEM)

✔ Think in steps, not formulas
✔ Draw mental diagram always
✔ Ask “WHAT is conserved?” first
✔ Never rush calculations
✔ Trust physics over memory

Next: Page 10 – Previous IIT/JEE Objective Questions + Complete Strategy

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