Year 10 Physics · Science ⏱ 60 min

Lesson 5: Momentum & Collisions — Ikeike me ngā Pahūtanga

Momentum is conserved in collisions. Students verify this with marble labs, analyse NZTA crash tests for road safety applications, and discover how patu design reflects deep understanding of impulse — Māori material science in action.

📎 Learning Intentions

Know | Mōhio

  • Momentum (p) = mass × velocity (p = mv), measured in kg·m/s.
  • Conservation of momentum: In a closed system (no external forces), total momentum before a collision equals total momentum after.
  • Impulse: The change in momentum of an object equals force × time (J = FΔt = Δp).
  • That crumple zones, airbags, and helmets all work by increasing collision time (Δt), reducing the peak impact force on the body (Impulse = FΔt, fixed Δp).

Demonstrate | Whakaatu

  • Calculate momentum before and after marble collisions and determine if momentum was conserved (within experimental error).
  • Use p = mv to solve 4 momentum calculation problems including NZTA road safety scenarios.
  • Explain in 2 sentences how crumple zones use impulse physics to save lives.

📐 Curriculum Alignment

NZC Science · Level 5 · Physical World
Explore, describe, and represent patterns and trends for everyday examples of physical phenomena, such as motion and forces.
Ministry of Education (2007). The New Zealand Curriculum, p. 60.

🌿 Whakatūwhera

He patu pounamu — he mōrehu i ngā rā o mua.

"A greenstone patu — a survivor from earlier times." Pounamu (greenstone) patu were engineered for maximum momentum transfer: dense material (high mass), smooth surface (reduced drag), short handle (controlled swing arc). Māori tohunga who designed these understood impulse physics intuitively — the hardest material ensures all momentum transfers to the target in the shortest time.

📚 Lesson Content

Part 1: Momentum — p = mv (10 min)

🔑 Momentum

p = mv   (momentum = mass × velocity)

Unit: kg·m/s (or N·s)

Momentum is a vector — direction matters. Rightward momentum is positive; leftward is negative.

Conservation of momentum: p_before = p_after (in a closed system)

Example: 0.02 kg marble moving at 2 m/s → p = 0.02 × 2 = 0.04 kg·m/s

Part 2: Marble Collision Lab (25 min)

Setup: Marble ramp, two identical marbles and one marble of different mass, ruler, balance, track on flat table.

Procedure (elastic collision — marbles bounce):

  1. Roll marble 1 (mass m₁) down ramp at a measured angle — record its speed at bottom using speed gate or video analysis (distance/time over 5 cm marked section).
  2. Place stationary marble 2 (mass m₂) at the bottom of the ramp.
  3. Release marble 1. Record speeds of both marbles after collision.
  4. Calculate momentum before and after. Are they equal?
Trialm₁ (kg)v₁ before (m/s)p₁ beforem₂ (kg)v₂ after (m/s)p_total afterConserved?
1: equal masses
2: heavy hits light
3: light hits heavy

Part 3: NZTA Crash Test Analysis (10 min)

A 1,500 kg car travelling at 14 m/s (50 km/h) hits a concrete barrier and stops in 0.1 seconds.

Same car with crumple zone extends collision time to 0.5 seconds:

Lesson: Crumple zones save lives by extending Δt. The same impulse (Δp) spread over more time → much smaller force.

Part 4: Patu Physics — Māori Material Science (5 min)

Pounamu (nephrite jade) patu: density ~3,000 kg/m³ — 3× denser than water, heavier than most stones. For a given swing velocity, a pounamu patu delivers far more momentum (p = mv) than a lighter wooden weapon. Additionally, jade is fracture-tough — it absorbs energy before breaking, unlike brittle materials. This is sophisticated materials engineering, not accident.

✅ Assessment

🎟 Exit Ticket

  1. A 0.15 kg rugby ball is kicked at 12 m/s. What is its momentum?
  2. Two identical 0.5 kg trolleys approach each other: Trolley A at 4 m/s east, Trolley B at 2 m/s west. They collide and stick together. What is their final velocity?
  3. Why does a thick gymnastics mat (slow stop over 0.3 s) injure you less than a concrete floor (fast stop over 0.01 s) if you fall from the same height?