Lesson 7 – Momentum & Collisions

Phase 3: Exam Mastery (Elite Level)
Page 13 – Objective 7 (Extreme Thinking & IIT Traps)

This page is designed for top-rank aspirants. Questions require mental visualization, logical elimination, and examiner psychology awareness.

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Q1. (IIT-JEE Advanced – Visualization)

A particle moves freely in space. Suddenly it explodes into two fragments of equal mass. Which statement is always correct immediately after explosion?

• A) Both fragments move perpendicular to original direction
• B) Both fragments have equal speeds
• C) Centre of mass continues in original direction
• D) Total kinetic energy remains unchanged

✔ Correct Answer: C

Elite Insight: COM ignores explosions.

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Q2. (Hidden Trap – Direction)

A ball strikes a smooth vertical wall and rebounds elastically. Which quantity reverses sign after collision?

• A) Speed
• B) Momentum
• C) Kinetic energy
• D) Mass

✔ Correct Answer: B

Trap: Speed stays same, direction flips.

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Q3. (Statement Analysis – IIT)

Consider the following statements:

(I) Momentum conservation depends on Newton’s third law (II) Energy conservation depends on Newton’s second law

• A) Both true
• B) Both false
• C) I true, II false
• D) I false, II true

✔ Correct Answer: C

Concept: Energy conservation comes from work–energy theorem.

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Q4. (Mental Math – Zero Calculation)

A particle breaks into three fragments. Which one of the following must be conserved?

• A) Velocity of each fragment
• B) Kinetic energy of each fragment
• C) Vector sum of momenta
• D) Speed of centre of mass

✔ Correct Answer: C

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Q5. (IIT-JEE – Deep Logic)

Why is centre of mass frame preferred in collision problems?

• A) It removes forces
• B) It simplifies conservation laws
• C) It increases accuracy
• D) It removes energy loss

✔ Correct Answer: B

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🧠 IIT Advanced Thinking Rules (Objective 7)

✔ COM thinking > particle thinking
✔ Direction matters more than magnitude
✔ Internal forces never change total momentum
✔ Explosion ≠ collision
✔ Visualize motion before calculating

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🏆 Elite Objective Section Completed

With Objective–7, students are trained to:
✔ Think like examiners
✔ Avoid panic traps
✔ Solve without formulas
✔ Handle IIT Advanced confidently

👉 Next: Final Revision (4 Parts) + Ultimate “How to Think in Exam Hall” Guide