Rotational Motion – Tough Objective Questions (Page 6)

Previous IIT-JEE / JEE Advanced Pattern

These objective questions are high-level, inspired directly from IIT-JEE Advanced & tough JEE Main papers. Each question includes method, trick, and examiner’s mindset.

                                       

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Q1. (IIT-JEE Advanced)

A disc is rotating freely with angular speed ω. Suddenly, a small mass is gently placed at its rim. What happens to angular speed?

Method:

• No external torque → angular momentum conserved
• Use L = Iω

Initial: L_i = (½MR²)ω
Final: L_f = (½MR² + mR²)ω'

ω' = (½MR² ω) / (½MR² + mR²)

Answer: Angular speed decreases

Examiner Trick: Students try τ = Iα (wrong here).

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Q2. (JEE Advanced)

A wheel rolls without slipping. At which point is velocity maximum?

Method:

Velocity = translational + rotational

Topmost point: v + v = 2v
Centre: v
Bottom: 0

Answer: Topmost point

Tip: Always draw velocity vectors.

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Q3. (IIT-JEE Advanced)

A rotating body has zero angular momentum. Which statement is correct?

Correct Thinking:

Angular momentum depends on reference axis.

Answer: Body may still be rotating

Trap: Students equate rotation with non-zero L.

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Q4. (IIT-JEE)

A disc and a ring roll down an incline without slipping. Which reaches bottom first?

Method:

Compare I / MR²

Disc: ½
Ring: 1

Answer: Disc

Shortcut: Smaller I/MR² → faster motion.

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Q5. (JEE Advanced)

A disc rolls from rough to smooth surface. Which quantity changes suddenly?

Method:

On smooth surface → friction = 0 → no torque

Answer: Angular acceleration becomes zero

Trap: Assuming angular velocity changes.

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Q6. (IIT-JEE Advanced)

Two bodies have same angular momentum. Which has greater rotational kinetic energy?

Method:

K = L² / (2I)

Answer: Body with smaller moment of inertia

Examiner Trap: Students think larger I → larger energy.

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Q7. (JEE Advanced)

A man standing on a rotating platform pulls his arms inward. What happens to angular speed?

Method:

No external torque → angular momentum conserved

Answer: Angular speed increases

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Q8. (IIT-JEE)

In pure rolling, work done by friction is:

Concept:

Point of contact has zero displacement.

Answer: Zero

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HOW TOPPERS SOLVE TOUGH OBJECTIVES

✔ First ask: Is torque zero?
✔ Prefer angular momentum in sudden events
✔ Compare I/MR² quickly
✔ Use energy only when friction work is zero
✔ Never assume rolling everywhere

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COMMON IIT EXAM TRAPS

❌ Applying τ = Iα when torque is zero
❌ Forgetting reference axis
❌ Ignoring rotational KE
❌ Assuming angular velocity always changes

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Next: Page 7 – Even Tougher Objective Questions (Multi-Concept)

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Advanced exam-oriented thinking series focusing on traps, execution, pressure handling, and topper strategies.

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