Year 10 Physics · Science ⏱ 60 min

Lesson 6: Friction & Air Resistance — Āuahi me te Takiwā

Friction and drag are the forces that slow us down — or keep us on the road. Students measure friction coefficients experimentally, optimise paper helicopters for minimal drag, and analyse how Team New Zealand shaved milliseconds off AC75 lap times through aerodynamic engineering.

📎 Learning Intentions

Know | Mōhio

That friction is a contact force opposing relative motion between surfaces, dependent on the normal force and the coefficient of friction (μ): F_friction = μ × N.
That air resistance (drag) is a force opposing motion through air, increasing with speed and surface area.
That terminal velocity occurs when drag force equals gravitational force (net force = 0, constant velocity).
That engineers at Emirates Team New Zealand spend millions reducing hull and wing drag — a 1% drag reduction can win a race at 50 knots.

Demonstrate | Whakaatu

Measure the friction force on 3 different surface combinations and calculate their coefficients of friction (μ).
Modify a paper helicopter's wing dimensions to maximise flight time (air resistance investigation).
Plot a speed vs. drag force graph and identify the terminal velocity region.

📐 Curriculum Alignment

NZC Science · Level 5 · Physical World — Physics Investigation

Carry out appropriate investigations, using relevant evidence to develop simple explanations.

Ministry of Education (2007). The New Zealand Curriculum, p. 28.

🌿 Whakatūwhera

He waka tere — ko te hau tōna hoa haere.

"A fast canoe — the wind is its travelling companion." Waka hulls were carved with remarkable hydrodynamic intuition: narrow at the bow to cut water (reduce drag), wider amidships for stability (manage normal force). Traditional knowledge embedded physics knowledge that Western science formalized centuries later.

📚 Lesson Content

Part 1: Friction Theory (10 min)

🔑 Friction

F_friction = μ × N

μ (mu) = coefficient of friction (no units — a ratio)

N = Normal force (N) — the force perpendicular to the surface (equals weight on flat ground)

Static friction: Prevents an object from starting to move. F_static_max = μ_s × N

Kinetic friction: Acts while an object slides. F_kinetic = μ_k × N (always slightly less than static)

Part 2: Sliding Blocks Friction Experiment (20 min)

Equipment: Wooden block (known mass), spring scale (force meter), various surfaces (tile, carpet, sandpaper, wax paper).

Procedure:

Place block on surface. Attach spring scale horizontally.
Pull slowly and steadily — record the force needed to maintain constant motion (kinetic friction).
Normal force N = mg (use g = 9.8 m/s²).
Calculate μ = F_friction / N for each surface.

Surface	Block mass (kg)	Normal force N (N)	Friction force F (N)	μ = F/N	Rough or smooth?
Tile (smooth)
Carpet
Sandpaper
Wax paper

Part 3: Paper Helicopter Investigation (15 min)

Students fold paper helicopters and modify wing dimensions (length, width, fold amount) to maximise flight time (greater air resistance = slower fall = more drag force = more useful).

Variables: Independent (wing width), Dependent (flight time), Controlled (drop height 2 m, same paper weight).

Plot wing width (x-axis) vs. average flight time (y-axis). What wing width produces maximum drag?

Part 4: AC75 Aerodynamics — Cup Yacht Engineering (10 min)

Team New Zealand's AC75 hull optimisation:

Foiling reduces drag by 80%: When the hull lifts out of water, skin friction (the largest drag component) drops from ~40,000 N to ~8,000 N.
Wingsail vs. cloth sail: The rigid wingsail generates more lift-to-drag ratio than a cloth sail — like a jet fighter wing vs. a bedsheet.
Speed-drag relationship: Drag ∝ v² — doubling speed quadruples drag. This is why the last 10% of speed gains require enormous power increases.

At terminal velocity: Thrust = Drag. Team NZ engineers calculate exactly what thrust their wingsail can generate, then design hull + foils to have exactly that much drag at peak racing speed. Every component is optimised.

✅ Assessment

🎟 Exit Ticket

A 5 kg box sits on a surface with μ = 0.3. What friction force acts when you try to slide it (g = 9.8 m/s²)?
A skydiver reaches terminal velocity at 55 m/s. Their mass is 80 kg. What is the drag force at terminal velocity?
Why does a waka hull that is narrower at the bow reduce drag — use the concept of streamlining in your answer.