Lesson 7 – Momentum & Collisions

Phase 3 – Final Revision
Page 2 – Formula Mastery & Application Logic

This page is designed for last-stage revision. Students should not memorise blindly, but understand WHEN and WHY each formula is used.

                                                        

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🔹 Formula 1: Linear Momentum

p = mv

✔ Vector quantity
✔ Direction matters
✔ Used in every collision / explosion problem

Common Mistake: Ignoring sign of velocity

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🔹 Formula 2: Law of Conservation of Momentum

Σp (before) = Σp (after)

✔ Valid when external force = 0
✔ Applies to collision, explosion, recoil

Exam Reminder: Wall & ground = external forces

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🔹 Formula 3: Coefficient of Restitution (e)

e = (relative speed of separation) / (relative speed of approach)

✔ 0 ≤ e ≤ 1
✔ e = 1 → elastic
✔ e = 0 → perfectly inelastic

Mistake: Applying e when collision does not occur

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🔹 Formula 4: Elastic Collision (1D)

v₁ = [(m₁ − m₂)/(m₁ + m₂)]u₁ + [2m₂/(m₁ + m₂)]u₂
v₂ = [2m₁/(m₁ + m₂)]u₁ + [(m₂ − m₁)/(m₁ + m₂)]u₂

✔ Use only when collision is elastic
✔ Saves time in IIT/JEE

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🔹 Formula 5: Height After Bounce

h₁ = e²h

✔ Vertical motion + rebound
✔ Time ratio = e

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🔹 Formula 6: Centre of Mass Velocity

V_COM = (Σmᵢvᵢ)/(Σmᵢ)

✔ Remains constant if no external force
✔ Explosion does not change COM motion

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🧠 Quick Decision Table

Collision → Momentum first
Explosion → COM + momentum
Bounce → e² rule
Elastic → KE conserved
Inelastic → KE lost

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📌 Next

Next page will include:
✔ 15 most repeated IIT/JEE results
✔ One-line memory tricks
✔ Mistake-proof revision list

👉 Next: Phase 3 – Final Revision (Page 3)