Lesson 7 – Momentum & Collisions

Phase 3: Exam Mastery (Ultimate Level)
Page 14 – Objective 8 (Integrated Thinking & Accuracy Control)

This page is meant for the final polishing stage. Questions integrate Momentum, Energy, Centre of Mass, Graphs, and examiner logic. These are the questions where rank-defining mistakes usually happen.

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Q1. (IIT-JEE Advanced – Integrated)

A particle moving with constant velocity suddenly explodes into two fragments. Which of the following quantities are definitely conserved immediately after explosion?

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

✔ Correct Options: A, C

Thinking: Explosion = internal forces only.

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Q2. (Graph-Based – JEE Main)

The momentum-time graph of a particle is a straight line with positive slope. Which conclusions are correct?

• A) Net force is constant
• B) Velocity is constant
• C) Acceleration is constant
• D) Kinetic energy increases linearly with time

✔ Correct Options: A, C

Trap: Linear momentum ≠ linear KE.

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Q3. (Collision + COM Frame)

In the centre-of-mass frame of reference, an elastic collision between two particles results in:

• A) Exchange of velocities
• B) Reversal of velocities
• C) Same speed before and after collision
• D) Zero total kinetic energy

✔ Correct Options: B, C

Key Idea: Directions change, magnitudes don’t.

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Q4. (Hidden Assumption – IIT)

A system of particles has zero net momentum. Which statement must be true?

• A) All particles are at rest
• B) Centre of mass is at rest
• C) Total kinetic energy is zero
• D) No forces act on particles

✔ Correct Answer: B

Trap: Internal motions can exist with zero total momentum.

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Q5. (Conceptual – Energy vs Momentum)

In which of the following situations is kinetic energy NOT conserved but momentum IS conserved?

• A) Elastic collision
• B) Perfectly inelastic collision
• C) Free fall
• D) Motion with friction

✔ Correct Answer: B

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🧠 Final Accuracy Control Rules (Objective 8)

✔ Always check: internal or external force?
✔ Explosion → momentum first, energy later
✔ Graphs → slope means force
✔ COM motion ignores internal chaos
✔ One calm read saves 4 marks

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🏁 Objective Series Completed

With Objective–8, students now have:
✔ Complete IIT-JEE objective coverage
✔ Zero-fear approach to traps
✔ Conceptual confidence
✔ Exam-hall clarity

👉 Next: Final Revision (4 Parts) + “How to Study & Revise This Chapter”