Lesson 7 – Momentum & Collisions

Stage 1: Intermediate Complete Notes
Page 8 – Recoil & Explosions

The law of conservation of momentum helps us understand several real-life phenomena such as recoil of a gun, rocket motion, and explosions.

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1️⃣ Recoil of a Gun

When a bullet is fired from a gun, the gun moves backward. This backward motion of the gun is called recoil.

✔ Initial momentum of system (gun + bullet) = 0
✔ Final momentum must also be zero

Mathematical expression:

m_bullet v_bullet + m_gun v_gun = 0

Since mass of gun is large, its recoil velocity is small.

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2️⃣ Numerical Example: Recoil

A bullet of mass 0.02 kg is fired with velocity 400 m/s from a gun of mass 4 kg. Find the recoil velocity of the gun.

Using momentum conservation:

(0.02 × 400) + (4 × v) = 0

v = −2 m/s

Negative sign indicates backward motion.

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3️⃣ Explosion

An explosion is a process in which a body breaks into two or more parts due to internal forces.

✔ Total momentum before explosion = 0
✔ Total momentum after explosion = 0

Even though kinetic energy increases, momentum remains conserved.

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4️⃣ Numerical Example: Explosion

A body at rest explodes into two pieces of masses 2 kg and 3 kg. If the 2 kg piece moves with velocity 6 m/s, find the velocity of the 3 kg piece.

Applying conservation of momentum:

2 × 6 + 3 × v = 0

v = −4 m/s

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5️⃣ Rocket Motion (Conceptual)

A rocket moves forward by ejecting gases backward. This motion is explained using momentum conservation.

✔ No external force needed in space
✔ Momentum of gases backward = momentum of rocket forward

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6️⃣ Important Exam Points

✔ Recoil velocity inversely proportional to mass
✔ Explosion increases kinetic energy
✔ Momentum conservation applies even when energy is not conserved

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📌 Page 8 Summary

✔ Recoil explained by momentum conservation
✔ Explosion involves internal forces only
✔ Rocket motion based on momentum exchange
✔ Important scoring applications

👉 Next page: Relative Velocity in Collisions & Special Cases