Year 10 Physics · Science ⏱ 60 min

Lesson 4: Newton's Third Law — Action and Reaction | Mahi me te Urupare

For every action force, there is an equal and opposite reaction force. Students build and race balloon rockets, optionally launch water rockets, and analyse how Rocket Lab's Rutherford engine uses this law to reach orbit.

📎 Learning Intentions

Know | Mōhio

Newton's Third Law: Whenever object A exerts a force on object B, object B exerts an equal and opposite force on object A. These are called action-reaction pairs.
That action-reaction pairs always act on different objects — they can never cancel each other out for a single object's motion.
That rocket propulsion, swimming, walking, and waka paddling are all powered by Newton's Third Law.

Demonstrate | Whakaatu

Build a balloon rocket that travels at least 2 m along a string and measure its speed (distance/time).
Identify the action force and reaction force in at least 4 scenarios, drawing force pairs correctly.
Explain in 3 sentences why a rocket can move in the vacuum of space (no air to push against) using Newton's Third Law.

📐 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

Ka tūkia te wai, ka huri te waka — ko te huri anō te ara whakamua.

"Strike the water, turn the waka — the turn itself is the forward path." A waka paddle pushes water backward (action); water pushes the paddle — and the waka — forward (reaction). This is Newton's Third Law enacted by every stroke.

📚 Lesson Content

Part 1: Action-Reaction Pairs — Theory (10 min)

🔑 Newton's Third Law

"For every action force, there is an equal and opposite reaction force."

Key rule: The action and reaction forces always act on different objects.

Situation	Action force	Reaction force
Waka paddle	Paddle pushes water backward	Water pushes paddle (waka) forward
Walking	Foot pushes ground backward	Ground pushes foot forward
Rocket engine	Engine ejects hot gas downward	Gas pushes rocket upward
Balloon rocket	Balloon pushes air backward	Air pushes balloon forward
Swimming	Arm pushes water backward	Water pushes arm (swimmer) forward

Part 2: Balloon Rocket Race (25 min)

Materials: Long balloons, string (3 m per team), straw, tape, timer.

Build procedure:

Thread string through straw and tie between two fixed points (chairs/poles), pulled taut.
Blow up balloon — do NOT tie it. Hold the opening closed.
Tape the balloon to the straw along the string.
Release and time how long the balloon travels the full length of string.
Calculate speed: distance / time.

Optimisation challenge: How can you make your rocket faster? Try: more/less inflation, balloon shape, string angle, tape placement. Record all trials with speed measurements.

Data table: Trial | Inflation level | String angle | Speed (m/s) | Notes on improvement

Part 3: Water Rocket Option (If facilities allow — 15 min)

Using commercial water rocket kit: fill rocket 1/3 with water, pressurize with pump to ~50 psi, launch vertically. Measure max height using clinometer or video analysis.

Connection: The compressed water is ejected downward (action). The bottle+remaining water is pushed upward (reaction). As water ejects, mass decreases → acceleration increases (F = ma link to Lesson 3).

Part 4: Rocket Lab Rutherford Engine Analysis (10 min)

Rocket Lab's Rutherford engine ejects liquid oxygen and RP-1 kerosene at 2,400 m/s through a nozzle. Each engine produces 24,000 N of thrust. Explain:

Action: Engine ejects 4 kg/s of hot gas at 2,400 m/s downward.
Reaction: Gas exerts 24,000 N of force on the rocket, upward.
In space (no air!): The rocket still works because it pushes its own fuel — it doesn't need anything to "push against" externally. This is the key insight that makes space travel possible.

✅ Assessment

🎟 Exit Ticket

When a gun fires a bullet, the gun recoils (kicks back). Name the action and reaction forces. Why does the bullet move faster than the gun?
A student claims: "Action and reaction forces cancel each other out, so nothing ever moves." What is wrong with this statement?
A waka paddler applies 300 N backward on the water. What force does the water apply on the paddle?