Lesson 7 – Momentum & Collisions

Phase 2: IIT / JEE Tough Problems
Page 5 – MCQ Traps & Elimination Mastery

This page trains students to solve IIT-JEE Advanced MCQs using logic, elimination, and physical insight, without long calculations. These are the questions where toppers save time.

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MCQ 1 (Impulse Trap)

A ball hits a rigid wall and rebounds. Which of the following quantities must be conserved?

• A) Kinetic energy
• B) Linear momentum of the ball
• C) Linear momentum of ball + wall
• D) Mechanical energy

✔ Correct Answer: C
Momentum is conserved only for the combined system.

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MCQ 2 (Coefficient of Restitution)

If the coefficient of restitution between two bodies is zero, which statement is correct?

• A) Bodies bounce back with same speed
• B) Bodies stick together
• C) Momentum is not conserved
• D) Kinetic energy is conserved

✔ Correct Answer: B
e = 0 ⇒ perfectly inelastic collision.

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MCQ 3 (Conceptual Elimination)

In which case is kinetic energy maximum after collision?

• A) e = 0
• B) e = 0.5
• C) e = 1
• D) e > 1

✔ Correct Answer: C
Elastic collision conserves kinetic energy.

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MCQ 4 (Explosion Logic)

A body at rest explodes into three pieces. Which of the following is always zero immediately after explosion?

• A) Momentum of each fragment
• B) Kinetic energy of system
• C) Velocity of centre of mass
• D) Speed of fragments

✔ Correct Answer: C
Centre of mass remains at rest.

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MCQ 5 (Zero Calculation)

Two particles collide elastically. Which quantity remains unchanged in magnitude for each particle?

• A) Velocity
• B) Momentum
• C) Speed in COM frame
• D) Kinetic energy

✔ Correct Answer: C
Speed in centre-of-mass frame remains same.

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🧠 IIT MCQ Elimination Strategy (Page 5)

✔ Check conservation laws first
✔ Eliminate options violating physics instantly
✔ Avoid unnecessary calculations
✔ Identify system clearly
✔ Trust centre-of-mass concepts

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✅ Phase 2 Completion Note

Phase 2 is now complete.

You have learned how to:
✔ Think before calculating
✔ Eliminate options quickly
✔ Handle toughest IIT traps calmly
✔ Save time without losing accuracy

👉 Next: Phase 3 – Exam Mastery (Topper Thinking, Traps, Strategy)