Lesson 2: Newton's First Law — Inertia | Mōhiotanga o te Āhuru
An object at rest stays at rest, an object in motion stays in motion — unless acted on by an unbalanced force. Students prove it three ways: tablecloth experiment, PhET simulation, and waka steering physics.
📎 Learning Intentions
Know | Mōhio
- Newton's First Law: An object remains at rest, or in uniform motion in a straight line, unless acted upon by a resultant (net) force.
- That inertia is an object's tendency to resist changes in its state of motion — proportional to its mass.
- That Polynesian navigators used their deep knowledge of ocean currents (steady forces) and trade winds (unbalanced forces) to maintain or change waka velocity.
Demonstrate | Whakaatu
- Correctly predict outcomes of 3 inertia scenarios using Newton's First Law before conducting experiments.
- Explain the tablecloth demonstration result in scientific language: identify the net force, state what inertia causes, predict what would change with a heavier plate.
- Make a video or annotated diagram showing one real-world NZ example of Newton's First Law.
📐 Curriculum Alignment
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
Ko te mea e tū ana, e tū tonu ana — ko te mea e oma ana, e oma tonu ana.
"That which stands, continues to stand — that which runs, continues to run." Newton formalized this truth in 1687. Polynesian navigators knew it 2,000 years earlier: once a heavy waka was in motion, only a sustained opposing force (paddling against the current) could slow it.
🎤 Kaiako Hook (5 min)
Do the classic tablecloth demo — pull a tablecloth quickly from under dishes. Then ask: "Why didn't the plates move? What force would have to act for them to move?" Then connect: "If you stopped paddling your waka on flat water, how far would it travel before stopping? What forces cause it to eventually stop?"
🎬 Watch
🎬 Kaiako: Select and embed a video here
Suggested search: "Newton first law of motion inertia Year 10 physics"
Search YouTube for a 5–10 min video on Newton's First Law (Law of Inertia). CrashCourse Physics ep. 3 or Khan Academy's Newton's laws series are excellent starting points.
Once you have selected a video, verify it is age-appropriate and plays correctly, then embed it using the iframe pattern in this file. Add data-oembed-verified="true" only after verifying via YouTube oEmbed.
Focus on: the formal statement of the law, what "net force" means, and why seat belts exist (an inertia safety application).
📚 Lesson Content
Part 1: Newton's First Law — Formal Statement (10 min)
🔑 Newton's First Law (Law of Inertia)
"An object will remain at rest, or in uniform motion in a straight line, unless acted upon by a resultant force."
Resultant force = net force = vector sum of all forces acting on an object.
If net force = 0: Object stays still, or moves at constant velocity (constant speed in a straight line).
If net force ≠ 0: Object accelerates — it speeds up, slows down, or changes direction.
Part 2: Tablecloth Experiment (20 min)
Setup: Place plastic cups (empty, then weighted with sand) on a smooth cloth. Pull the cloth sharply.
Predictions (before each trial): Will the cups move? Which way? How far?
| Trial | Mass of cup | Pull speed | Predicted result | Actual result | Why? (Newton's 1st Law) |
|---|---|---|---|---|---|
| 1 | Empty (50 g) | Slow | |||
| 2 | Empty (50 g) | Fast | |||
| 3 | Full (200 g) | Fast | |||
| 4 | Full (200 g) | Slow |
Key question: Why does a faster pull reduce cup movement? (The friction force acts for a shorter time — less impulse transferred to the cup.)
Part 3: PhET Simulation (10 min)
Open: PhET "Forces and Motion: Basics" (phet.colorado.edu — free, browser-based). Set friction to zero. Apply a force then remove it. Observe: the object continues at constant velocity. Now add friction. What happens?
This simulation lets students directly test the First Law in a controlled, frictionless environment — impossible in the physical classroom.
Part 4: Waka Inertia Application (10 min)
A 3-tonne waka hourua moving at 15 knots (7.7 m/s) on flat water:
- Which forces are acting on it? Thrust (paddles/sail), Drag (water friction), Buoyancy, Weight.
- If thrust stops: Only drag remains as a net backward force → the waka decelerates.
- Why is a heavier waka harder to stop? Greater mass = greater inertia = more force needed to change velocity.
Polynesian navigators chose waka sizes deliberately — large waka maintained speed through swells (high inertia) while smaller waka were more maneuverable (lower inertia). This is applied physics, millennia before textbooks.
Inertia Video Project (Take-Home Assessment)
Students film a 30-second video of one real-world inertia example in their home or community. They narrate: "This is [describe scenario]. According to Newton's First Law, the [object] tends to [remain at rest / keep moving] because [net force explanation]."
✅ Assessment
🎟 Exit Ticket
- A pōhiri guest sits in a bus that suddenly brakes. Which direction does the guest lurch? Name the law and explain in 2 sentences.
- On the International Space Station (no air, no friction), an astronaut pushes off a wall. What happens to them? Use Newton's First Law.
- Why do larger waka need more sustained paddling force to accelerate than smaller waka?