Lesson 7 – Momentum & Collisions

Phase 3: Exam Mastery
Page 9 – Objective 3 (Multi-Correct & Assertion–Reason)

This page trains students to handle multi-correct MCQs and assertion–reason questions, where partial understanding leads to negative marking in IIT-JEE Advanced.

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MCQ 1 (Multi-Correct – IIT-JEE Advanced)

In a one-dimensional collision between two bodies, which of the following statements are correct?

• A) Momentum is conserved if no external force acts
• B) Kinetic energy is always conserved
• C) Coefficient of restitution can be zero
• D) Relative velocity of separation depends on e

✔ Correct Options: A, C, D

Trap: KE is conserved only in elastic collisions.

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MCQ 2 (Multi-Correct – Explosion)

A body initially at rest explodes into several fragments. Which of the following are always true?

• A) Total momentum remains zero
• B) Total kinetic energy increases
• C) Velocity of centre of mass remains unchanged
• D) Individual fragment momenta are zero

✔ Correct Options: A, C

Examiner Trick: KE may increase or decrease — not fixed.

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Assertion–Reason 1

Assertion (A): In an elastic collision, kinetic energy is conserved.

Reason (R): In elastic collisions, the coefficient of restitution is 1.

✔ Correct: Both A and R are true and R is the correct explanation of A.

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Assertion–Reason 2

Assertion (A): During a collision with a wall, momentum of the ball is not conserved.

Reason (R): The wall exerts an external force on the ball.

✔ Correct: Both A and R are true and R is the correct explanation of A.

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MCQ 3 (Multi-Correct – COM Frame)

In the centre-of-mass frame of a two-particle system:

• A) Total momentum is zero
• B) Velocities of particles are equal and opposite
• C) Speeds of particles change after elastic collision
• D) Kinetic energy is minimum

✔ Correct Options: A, B, D

Key Insight: Elastic collision changes direction, not speed (in COM frame).

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🧠 Objective-3 Exam Tips

✔ Multi-correct → verify each option independently
✔ Never assume KE conservation by default
✔ Explosion questions → start from momentum = 0
✔ Assertion–Reason → check explanation link, not just truth
✔ One wrong tick = full negative → be calm

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📌 Next

Next page will include:
✔ Tough previous year numericals (objective type)
✔ Mixed single + multi-correct
✔ Final speed-boosting strategies

👉 Next: Objective 4 (Page 10)