Lesson 4: Momentum & Impulse in One-Dimensional Collisions
Demonstrating understanding of mechanical principles, motion, forces, and energy for NCEA Level 2 Physics.
🎯 Ngā Whāinga Akoranga | Learning Intentions
Linear momentum (p = mv), impulse (Delta p = F Delta t), conservation of momentum in isolated systems, and elastic vs inelastic collisions.
Calculate vector momentum before and after one-dimensional collisions, and explain crumple zones and airbags using impulse-force-time relationships.
🎥 Media Anchor & Pedagogical Scaffold
Momentum Conservation & Impulse Mechanics
Video Clip: Momentum (Collisions: Bowling Balls) — conservation of momentum worked through bowling-ball examples, by ExamQA (Runtime: 5m 23s). Note: a UK GCSE resource. The one-dimensional momentum physics is the same as ours; the exam framing is not.
🧠 1. Before Viewing (Activate & Predict)
Why do car crumple zones and airbags save lives during high-speed collisions by extending impact time?
👁️ 2. During Viewing (Watch With a Job)
- Define: write the formula for momentum. What are the units? Is momentum a scalar or vector quantity?
- State the law: what does conservation of momentum say happens in a closed system? Restate it in your own words.
- Calculate: a 0.5 kg ball moving at 4 m/s strikes a stationary 0.5 kg ball. Using conservation of momentum, what is the combined speed if they stick together?
🗣️ 3. After Viewing & Kaiako Move (Process & Apply)
Kaiako Move: Use dynamics air-track or marble collision ramps to demonstrate 1D momentum conservation hands-on.
Immediate Task: Complete Section 4 of your Mechanics Portfolio: One-Dimensional Collision Analysis & Safety Impulse Justification.
⚡ Whakaoho | Do Now: Physics Mechanics Recall (10 mins)
An airbag does not reduce your momentum change. You still go from full speed to zero — the change is identical with or without it.
Two minutes: if Δp is the same either way, what exactly is the airbag changing? Look at Δp = FΔt and say which quantity it stretches and which it therefore shrinks. That single rearrangement explains airbags, crumple zones, catching a ball with soft hands, and why gymnasts land on mats.
📖 Activity 1: Mechanical Investigation & Vector Problem Solving (25 mins)
Conserve it (15 min). Solve the three collision problems below — one elastic, one inelastic, one explosion. Momentum is a vector, so assign directions before you start and keep the signs. Then check kinetic energy before and after, and state which collisions conserved it and which did not.
Problem 1 (elastic). A 0.50 kg dynamics cart rolling east at 2.4 m s⁻¹ collides elastically with an identical stationary cart on the same line. Find the velocity of each cart after the collision, and show kinetic energy is conserved.
Problem 2 (inelastic). A 0.150 kg ball moving east at 4.0 m s⁻¹ collides with a 0.250 kg ball at rest, and they stick together. Find their common velocity after impact, and show kinetic energy is not conserved.
Problem 3 (explosion). A 60 kg kaiako sitting at rest in a stationary waka throws a 2.0 kg kete of kaimoana horizontally at 6.0 m s⁻¹. Find the kaiako and waka's recoil velocity, including its direction.
Impulse in practice (10 min). A 70 kg driver travelling at 15 m s⁻¹ is brought to rest. Calculate the average force on the driver if the stop takes 0.1 s versus 0.5 s. Compare the two numbers, then explain a crumple zone using them rather than describing it.
📝 Activity 2: Level 2 Physics Portfolio Task & Merit/Excellence Scaffolding (20 mins)
Portfolio — Section 4. Submit: (1) three collision problems with directions assigned and momentum conservation shown; (2) your kinetic energy check identifying elastic and inelastic; (3) the two impulse forces calculated and a crumple-zone explanation built on those numbers.
🏫 Kaiako Planning & Pedagogy Notes
NCEA Level 2 Alignment: Direct preparation for Level 2 Physics (Demonstrate understanding of mechanics). Emphasise linking mathematical working directly to physical concept explanations for Merit/Excellence grades.
Scope honesty (AS91171 v2, Explanatory Note 4): the standard assesses change of momentum, impulse and conservation of momentum in one dimension only. Two-dimensional collision analysis is not assessed in this standard — if you extend into 2D with able classes, label it as extension work, not exam preparation.