Momentum & Collisions
Year 11 (IGCSE) 🚀 Forces & Motion Apply conservation of momentum to elastic and inelastic collisions.
💥 Conservation of Momentum
In a closed system (no external forces), total momentum before equals total momentum after. This is one of the most powerful laws in physics!
u = initial velocity, v = final velocity. Remember: velocity is a vector — direction matters! Use + for one direction and − for the other.
Before: p = 3×4 + 1×0 = 12 kg m/s
After: 3×1 + 1×v = 12 → v = 9 m/s
🎱 Elastic vs Inelastic Collisions
Two types of collision: in both, momentum is conserved, but kinetic energy may or may not be.
| 🏷️ Type | ⚡ KE conserved? | 🔍 Example |
|---|---|---|
| ✅ Elastic | Yes — KE before = KE after | Snooker balls, gas molecules |
| ❌ Inelastic | No — some KE lost as heat/sound | Car crash, clay hitting wall |
| ❌❌ Perfectly inelastic | Maximum KE lost — objects stick together | Two lumps of plasticine |
🚀 Explosions and Recoil
When two objects explode apart (initially stationary), the total initial momentum is zero — so the momenta after must be equal and opposite.
Momentum of bullet = 0.02 × 400 = 8 kg m/s (forward)
Gun recoil speed = 8 ÷ 2 = 4 m/s (backward)
p = mv | Conservation: m₁u₁ + m₂u₂ = (m₁+m₂)v (perfectly inelastic)