Lesson 7 – Momentum & Collisions

Phase 3: Exam Mastery
Page 11 – Objective 5 (Final Traps & Confidence Builder)

This page contains final-level IIT/JEE objective questions that test calm reading, deep understanding, and mistake control. These questions are usually answered correctly by top 5–10% students.

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Q1. (IIT-JEE Advanced – Trap)

Two particles collide. After collision, both particles move in the same direction with the same speed. Which of the following must be true?

• A) Collision is elastic
• B) Collision is perfectly inelastic
• C) Momentum is not conserved
• D) Kinetic energy is conserved

✔ Correct Answer: B

Key Insight: Same final velocity ⇒ e = 0.

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Q2. (JEE Main)

A ball is dropped from a height h and rebounds to height h/4. The coefficient of restitution between the ball and ground is:

• A) 1/2
• B) 1/4
• C) √(1/4)
• D) √(1/2)

✔ Correct Answer: C

Rule: h₁ = e²h ⇒ e = √(h₁/h)

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Q3. (IIT-JEE Advanced)

In a system of particles, which statement is always true in the absence of external forces?

• A) Velocity of every particle is constant
• B) Momentum of every particle is constant
• C) Velocity of centre of mass is constant
• D) Kinetic energy is constant

✔ Correct Answer: C

Examiner Trap: Individual particles may accelerate internally.

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Q4. (IIT-JEE)

A bullet passes through a block without embedding. Which of the following quantities must be conserved during the interaction?

• A) Mechanical energy
• B) Kinetic energy of bullet
• C) Momentum of bullet + block
• D) Speed of bullet

✔ Correct Answer: C

Golden Rule: Momentum survives many situations — energy doesn’t.

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Q5. (Conceptual – Zero Calculation)

Which collision has maximum loss of kinetic energy?

• A) Elastic collision
• B) Partially elastic collision
• C) Perfectly inelastic collision
• D) Super elastic collision

✔ Correct Answer: C

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🧠 Final Objective Strategy (Before Exam)

✔ Read final velocities carefully — they reveal e directly
✔ If heights are given → think e² immediately
✔ Explosion → COM velocity unchanged
✔ Momentum first, energy second
✔ Calm reading beats fast calculation

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✅ Objective Section Completed

You have now completed:
✔ All objective patterns
✔ All IIT/JEE traps
✔ Speed-solving techniques
✔ Confidence-building practice

👉 Next: Final Revision (4 Parts) + How to Study This Chapter in Last 7 Days