Offline Classroom Activities — Science 9
Term 1 · Week 3 · CG Quarter 1, competencies 4–5 · Action–reaction pairs
Three individual and two group activities. No electricity, nothing to buy, no laboratory equipment. All five are written for a room of 45 or more with fixed desks.
One caution before any of these. Two of these activities depend on a difference in mass between two learners, and that is the point of them. Never announce, ask for or write down anybody's weight, and never say the fat one or the heavy one. Say "the two of you, please" and let the physics be visible without anyone being labelled. If a class has a learner who would be hurt by being chosen for this, choose two learners of similar size and use a loaded bag on one chair instead — a sack of books is a mass difference nobody has to feel.
Activity 1 — Push the Wall (individual)
| Time | 15 minutes |
| Materials | A wall. A smooth patch of floor. Socks, if the floor allows |
| Preparation | Two minutes: clear a metre of floor beside a wall and check it is not slippery enough to be dangerous. |
Instructions
- Stand facing a wall, an arm's length away, feet together, in socks on a smooth floor.
- Push the wall. Push properly.
- Write down what happened to you.
- Write down what happened to the wall.
- Answer: you were the only person pushing. So why did you move?
- Write both forces as full sentences, in the form A pushes B.
- Answer: which object does each of those two forces act on?
Why this one is first
Everything else this week involves two people, and a learner can watch two people push each other and still believe one of them was the active one and the other was passive. Here there is only one person and they still get pushed back. The wall did not decide to do anything, and the learner still slid.
Step 5 is the whole activity. Steps 6 and 7 are the habit the week is trying to install.
Differentiation
| One step down | Steps 1 to 5 only, answered aloud to the teacher rather than written. |
| One step up | Do it again pushing at an angle, upwards or downwards. Which way do you go? Predict before you push. The answer is opposite to the way you pushed, and it surprises people. |
Assessment
| Seen | What it means |
|---|---|
| Both forces written in full with the object each acts on | The competency is met |
| The wall pushed me back and nothing more | The idea is there and the naming habit is not. Ask for step 6 aloud |
| I moved because I pushed off | Not yet an interaction. Ask: pushed off what? And what did it do to you? |
| The wall did nothing | The most useful wrong answer in the week. If the wall did nothing, what moved you? |
Activity 2 — Two Masses, One Push (group)
| Time | 35 minutes |
| Group size | 5 — works at 4 to 6 |
| Materials | Two chairs that slide. Chalk to mark the floor. Something to measure with — a tape, a metre stick, or counted floor tiles |
| Preparation | Ten minutes: find two chairs that slide on your floor, clear three metres, and mark a starting line. |
Roles — everybody has one
| Role | What they do | How many |
|---|---|---|
| Pushers | Sit on the chairs, feet off the floor, and push off. Only one of them pushes each time | 2 |
| Marker | Chalks where each chair came to rest, before anybody stands up | 1 |
| Measurer | Measures both distances from the starting line and reads them out | 1 |
| Recorder | Writes both numbers in the table. Nothing is written that the Measurer did not say | 1 |
Roles rotate after every two runs, so everybody pushes and everybody measures.
Instructions
- Both pushers sit facing each other, chair backs to the marked line, feet off the floor.
- One of them pushes. Only one. The other does nothing.
- The Marker chalks both stopping points before anybody stands up.
- The Measurer reads both distances. The Recorder writes them down.
- Run it six times: twice with two similar-sized learners, twice with two of visibly different size, and twice with a loaded bag on one of the chairs.
- Each learner writes: the six pairs of numbers, and one sentence saying what the pattern is.
- Answer the two questions below.
The two questions
a. In every run, which chair was pushed harder?
Neither. The two forces were equal in every single run, including the ones where the distances were completely different.
b. Then why were the distances different?
Different masses. The same force gives a smaller acceleration to a larger mass, so it travels less. Equal forces, unequal motion — and the numbers in the table are the class's own evidence for it.
Push this until it is uncomfortable. Most classes will write the heavier one pushed less hard, because that is the only explanation available if you have not separated force from acceleration. Point at run 1, where the two learners were the same size and the distances matched, and ask what changed between run 1 and run 3. The only thing that changed was mass.
Rubric — 12 marks
| Criterion | 3 | 2 | 1 | 0 |
|---|---|---|---|---|
| Six runs completed | Six, all three conditions, both distances each time | Four or five | Two or three | Fewer than two |
| Roles rotated | All members pushed and all measured | Rotated at least twice | Rotated once | One person did it all |
| The data recorded | All twelve distances, in a table | Most | Some | Not recorded |
| The conclusion | States that the forces were equal and the masses were not, in their own words | States that the distances differ with size | Records the numbers only | Absent, or the heavier one pushed harder |
Activity 3 — The Rope Never Lies (group)
| Time | 30 minutes |
| Group size | 8 or more — two teams of at least four |
| Materials | A length of rope. A cloth or chalk mark for the middle |
| Preparation | Five minutes: check the rope for frays, tie no loops in it, and find a space outside or a corridor. Nobody wraps rope around a hand or wrist. |
Roles — everybody has one
| Role | What they do | How many |
|---|---|---|
| Pullers | Pull. That is the whole job and it is not a small one | 8+ |
| Judge | Watches the centre mark and calls the winner. Nobody argues with the Judge | 1 |
| Feet Watcher | Watches the feet of both teams, not the rope, and reports what they saw | 1 |
| Predictor | Writes, before each round, which team will win and why | 1 |
| Recorder | Writes the result of each round and the Feet Watcher's report | 1 |
Instructions
- Round 1: two teams as evenly matched as you can make them. Pull. Record.
- Round 2: same teams, but one team takes their shoes off and stands in socks.
- Round 3: shoes back on, but one team stands on a smooth surface if you have one — a polished corridor, or a plastic sheet.
- After each round the Feet Watcher reports what the feet were doing.
- Each learner writes: who won each round, and what changed between rounds.
- Answer the three questions below.
The three questions
a. How does Team A's pull on the rope compare with Team B's?
They are exactly equal, in every round, including the ones with a clear winner. That is the third law and it is not negotiable.
b. Then what decides the winner?
The friction between each team's feet and the ground. Round 2 and Round 3 are the evidence: the rope did not change and the result did.
c. So why do heavier teams usually win?
More weight presses their feet harder against the ground, which makes more friction available. Mass helps — but by way of the ground, not by making their pull on the rope larger. This is the distinction the whole activity exists to make.
⚠️ Safety, and it is not a formality. No rope round a hand, wrist or waist. Both teams stop on the word stop, not on the winning pull. Do not run this on concrete with learners in bare feet.
Rubric — 12 marks
| Criterion | 3 | 2 | 1 | 0 |
|---|---|---|---|---|
| Three rounds run and recorded | All three, with the change in each recorded | Two | One | None |
| Prediction made first | Written before each round, with a reason | Written before, no reason | Written after | None |
| Everybody worked | All five roles carried out, pullers included | Four roles | Three | The teams pulled and nobody watched |
| Question (a) answered | Equal, always — stated clearly and defended when challenged | Equal | Nearly equal | The winners pulled harder |
Activity 4 — Four Forces, Two Diagrams (individual)
| Time | 25 minutes |
| Materials | Paper and pencil. A book and a table, which the room already has |
| Preparation | None. Put a book on a table at the front where everybody can see it. |
Instructions
- Put your own book on your own desk. Look at it.
- Write the two forces acting on the book. Full sentences, both objects named.
- Write the partner of each of those two forces.
- You now have four forces. For each one, write the single object it acts on.
- Draw two separate diagrams: one showing every force acting on the book, and one showing the forces acting on the table and on the Earth.
- Answer: are the two forces on the book an action–reaction pair? How do you know?
- Answer: the book is not moving. Which of your four forces explains that, and which does not?
The answers
| Force | Acts on |
|---|---|
| The Earth pulls the book down | the book |
| The table pushes the book up | the book |
| The book pulls the Earth up | the Earth |
| The book pushes the table down | the table |
Step 6: No. The first two both act on the book — the same object — so they are balanced forces, not an action–reaction pair. They cancel, which is why the book is still, and pair forces never cancel.
Step 7: The two forces on the book explain why it is not moving. The other two act on the Earth and on the table and have nothing to do with the book's motion at all — which is exactly why they are so easy to forget.
Why the two diagrams matter more than the answer
A single diagram with four arrows on it teaches the misconception, because every arrow looks like it belongs to the same situation. Splitting them by object is the competency: CG competency 5 asks learners to draw force diagrams to explain how the pairs affect the motion of objects, and a diagram that mixes objects cannot explain any object's motion.
Differentiation
| One step down | Steps 1 to 4 only, using the table in worksheet-remediation.html task 3, which supplies the four forces and asks only for the objects. |
| One step up | Now put a second book on top of the first. How many forces are there, and how many pairs? Draw three diagrams. |
Assessment
| Seen | What it means |
|---|---|
| Two diagrams, four forces, objects named, step 6 answered no with the two-object reason | Full understanding |
| Four forces correct, one diagram with all four on it | The physics is there and the diagramming is not. That is a ten-second fix and worth making |
| Step 6 answered yes | The diagnostic. Send them to step 4 and have them read their own column of objects |
| Only two forces found | The partners have not been looked for. Ask what the book is doing to the table |
Activity 5 — The Balloon and the Newspaper (individual)
| Time | 20 minutes |
| Materials | One balloon per learner, or one per pair. A drinking straw and tape, if you have them; a length of string across the room |
| Preparation | Ten minutes: tie a string between two chairs across the room, waist height. Thread the straws on before tying the second end. |
Instructions
- Tape your inflated balloon to a straw threaded on the string. Hold the neck closed.
- Predict, in writing, which way it will go and why.
- Let go.
- Write the two forces, in full sentences: what does the balloon push, and what pushes the balloon?
- Answer: which object does each force act on?
- Then read the story below and answer the question at the end.
The story
In 1920, a New York newspaper printed an editorial about a scientist named Robert Goddard, who had written that a rocket could travel through space. The editorial said he was wrong, because in space there is a vacuum and a rocket would have nothing to push against. It said he lacked the knowledge taught daily in high schools.
In 1969, as three astronauts travelled to the Moon, the same newspaper printed a correction.
Your task: write the correction the newspaper should have printed in 1920. Three sentences, naming both objects in the pair.
The answer
A rocket pushes on its own exhaust gas, and the gas pushes back on the rocket. Both objects are part of the rocket's own system, and nothing outside is needed at all. Air is not something a rocket pushes against — it is something a rocket has to push through, so a rocket in vacuum works better, not worse.
Why the newspaper matters
It is the most famous public example of exactly the misconception this week is written against, made by professional writers who had all been taught the third law at school. Saying that out loud is worth more than another worked example, because it tells a class that getting this wrong is ordinary and that getting it right is an achievement rather than a formality.
Assessment
| Seen | What it means |
|---|---|
| Both forces named with objects, and a correction that says its own exhaust | Full understanding |
| Correct forces, correction says the reaction pushes it | The pair is understood, the naming habit is not |
| Correction says there is air in space | The physics has been rescued by inventing a fact. Worth a gentle public correction |
| A rocket cannot work in space | Agrees with the 1920 editorial. Go back to step 4 — what did the balloon push? |
If there is no rope, no balloon and no smooth floor
- No balloon — Activity 5 works as the reading and writing task alone, with the class watching one teacher-held balloon released across the room. The story carries it.
- No straw or tape — let the balloon go free. It goes in an untidy loop instead of a straight line, and the physics is identical: it pushed air one way and went the other.
- No rope — Activity 3 becomes two learners pushing palm to palm in socks, with the class watching whose feet slide. The friction point survives intact and the teams do not.
- No smooth floor for Activity 1 or 2 — sit the pushers on folded sacks, or put a sheet of card under each chair. If nothing slides at all, Activity 2 can be run as a thought experiment with the chairs held and released by a third learner, and the measurements dropped. It is much weaker, and it is honest to tell the class that the numbers are the missing part.
- A class where comparing two learners' sizes would be unkind — use one chair loaded with a bag of books for every run and never pair two learners by size. The mass difference is the same and nobody is the demonstration.
Part of the E-turo MATATAG artifact set. Companion files:
syllabus.md · lesson-plan.md ·
lesson-plan-ilaw.md · worksheet.html