Lesson 7 – Momentum & Collisions

Phase 3 – Final Revision
Page 3 – High-Return Results & Memory Tricks

This page contains examiner-favourite results that appear repeatedly in Board, JEE Main, and JEE Advanced. Memorising and understanding these saves time + marks.

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🔹 Result 1: Equal Mass Elastic Collision

Two equal masses collide elastically → they exchange velocities.

Memory Line: “Same mass, same speed swap.”

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🔹 Result 2: Light Mass Hits Heavy Mass

Lighter mass rebounds with nearly same speed (elastic collision).

✔ Direction reverses
✔ Speed almost unchanged

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🔹 Result 3: Heavy Mass Hits Light Mass

Heavy mass continues forward with small change in velocity.

Physical Sense: Heavy objects are hard to stop.

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🔹 Result 4: Perfectly Inelastic Collision

✔ Bodies stick together
✔ Maximum loss of kinetic energy
✔ Momentum conserved

Exam Line: “Stick together → KE loss maximum.”

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🔹 Result 5: Explosion at Rest

If system is initially at rest → vector sum of momenta after explosion = 0

Shortcut: p₁ + p₂ + p₃ = 0 (always)

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🔹 Result 6: Centre of Mass Motion

In absence of external force, COM moves with constant velocity.

✔ Explosion does not affect COM motion
✔ Internal forces cancel

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🔹 Result 7: Bounce from Ground

Height after bounce = e² × initial height

✔ Time ratio = e
✔ Velocity ratio = e

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🧠 Examiner Favourite Traps

✔ KE assumed conserved ❌ ✔ Direction ignored ❌ ✔ Explosion treated as collision ❌ ✔ COM misunderstood ❌ ✔ Height vs time mixed ❌

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🔹 One-Line Ultra-Fast Revision

Momentum survives where energy fails.
COM ignores internal chaos.
Elastic → no KE loss.
Inelastic → KE loss maximum.
Direction matters more than magnitude.

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📌 Next

Next page will include:
✔ 7-Day revision plan
✔ 1-Day before exam checklist
✔ How toppers revise this chapter

👉 Next: Phase 3 – Final Revision (Page 4)