Lesson 7 – Momentum & Collisions

Phase 3: Exam Mastery
Page 15 – Objective 9: How to Tackle Problems (Tips–1)

This page explains HOW to think when you see a problem. Most students fail not because of formulas, but because they start wrongly.

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🔹 Tip 1: Identify the EVENT First

Before writing any equation, ask: Is this a collision, explosion, rebound, or smooth motion?

✔ Collision → think momentum & e
✔ Explosion → think COM & momentum
✔ Bounce → think e² (height), e (time)

Mistake: Directly applying energy in collision problems.

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🔹 Tip 2: Check External Forces (Most Critical)

Ask immediately: Is there any external force during interaction?

✔ No external force → momentum conserved
✔ External force present → momentum NOT conserved

Exam Trap: Wall, ground, friction act as external agents.

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🔹 Tip 3: Decide the Frame of Reference

Many problems become simple if shifted to: Centre of Mass frame.

✔ In COM frame → total momentum = 0
✔ Elastic collision → speeds unchanged

Use COM frame when: equal masses, symmetric collisions, velocity exchange problems.

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🔹 Tip 4: Never Mix Energy & Momentum Blindly

Momentum conservation applies in more situations than energy.

✔ Use momentum first
✔ Use energy only if explicitly elastic or smooth motion

Golden Rule: Energy is picky. Momentum is loyal.

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🔹 Tip 5: Watch the Direction (Sign Discipline)

Many students lose marks due to sign mistakes.

✔ Choose one direction positive
✔ Stick to it till final answer
✔ Negative velocity = direction change, not error

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🧠 Exam Hall Mindset (Tips–1 Summary)

✔ Pause 10 seconds before writing
✔ Identify event & forces
✔ Choose correct conservation law
✔ Visualize motion direction
✔ Then write equations calmly

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📌 Next

Next page:
✔ How to avoid calculation traps
✔ Speed shortcuts
✔ Option elimination techniques

👉 Next: Objective 10 – How to Tackle Problems (Tips–2)