1.6 Momentum & Impulse
Extended Tier Only - The Physics of Collisions
📋 Syllabus: 1.6.1 - 1.6.4
🎬 Hook & Why This Matters
Ever wondered why a car crash at 60 mph is so much more dangerous than at 30 mph? Or why airbags save lives? The answer lies in momentum - a quantity that combines mass and velocity. When things collide, momentum is conserved, but the forces involved depend on how long the collision lasts. This is the physics behind seatbelts, crumple zones, and crash helmets.
📐 1.6.1: What is Momentum?
Definition (Mark Scheme Keywords):
Momentum = mass × velocity
p = mv
p
momentum (kg m/s)
m
mass (kg)
v
velocity (m/s)
⚠️ CRITICAL: Momentum is a VECTOR
Direction matters! Always define a positive direction. Velocity (and therefore momentum) in the opposite direction is negative.
⚖️ 1.6.2: Conservation of Momentum
The Principle (Mark Scheme):
"In a closed system, the total momentum before a collision equals the total momentum after the collision."
Total p(before) = Total p(after)
m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂
🔬 INTERACTIVE: Collision Laboratory
🔴 Red Trolley
🔵 Blue Trolley
📊 STEP-BY-STEP Calculation:
⬅️ BEFORE Collision:
Step 1: Calculate Red Trolley Momentum
p₁ = m₁ × u₁
p₁ = 2 kg × 3 m/s
p₁ = 6.00 kg m/s
Step 2: Calculate Blue Trolley Momentum
p₂ = m₂ × u₂
p₂ = 3 kg × (-2) m/s
p₂ = -6.00 kg m/s
Step 3: Total Momentum BEFORE
Total p(before) = p₁ + p₂
Total p(before) = 6.00 + (-6.00)
Total p(before) = 0.00 kg m/s
⚡ 1.6.3: Impulse - Force × Time
Definition & Formula:
Impulse = Change in Momentum
Ft = Δ(mv)
Ft = mv - mu
F
force (N)
t
time (s)
m
mass (kg)
v, u
final, initial velocity (m/s)
💡 Key Insight:
For the SAME change in momentum, if you increase the time, you decrease the force. This is why safety features work!
🛡️ 1.6.4: Safety Features - The Egg Drop Demo
🥚 THE EGG DROP EXPERIMENT
🚗 Real-World Safety Features:
🛡️ Airbags
Increase contact time during collision → reduces force on head/chest
🔗 Seatbelts
Stretch slightly to increase stopping time → reduces force on body
🚙 Crumple Zones
Car body deforms to increase collision time → reduces force on passengers
🪖 Crash Helmets
Foam padding increases impact time → reduces force on skull
⚠️ EXAMINER TRAPS - Don't Lose Marks!
🎯 Trap 1: Forgetting Direction
❌ "The momentum is 50 kg m/s"
✓ "The momentum is 50 kg m/s to the right" or use negative for left
🎯 Trap 2: Wrong Units
❌ Momentum in "N" or "m/s"
✓ Momentum MUST be in kg m/s
🎯 Trap 3: Safety Features
❌ "Airbags reduce momentum"
✓ "Airbags increase the time over which momentum changes, which reduces the force"
🎯 Trap 4: Conservation Conditions
❌ "Momentum is always conserved"
✓ "Momentum is conserved in a closed/isolated system (no external forces)"
✓ Syllabus Coverage Checklist
1.6.1: Define momentum
p = mv (vector quantity)
1.6.2: Conservation principle
Total momentum before = after
1.6.3: Define impulse
Ft = Δ(mv)
1.6.4: Safety features
Increase time → decrease force
🎓 Master momentum and you'll understand why the universe conserves it!
Next: Forces and Energy!
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