Lesson Exemplar — Science 9

Term 1 · Week 3 · Day 1 · Domain: Force, Motion, and Energy

This is one day. For the whole of Week 3 in the format DepEd prescribes for filing, see lesson-plan-ilaw.md.

I. CURRICULUM CONTENT, STANDARDS, AND LESSON COMPETENCIES

A. Content Standards

Quoted from the curriculum guide (DepEd, August 2023, p. 55), Grade 9 Quarter 1, content standard 1:

Newton's laws explain and predict how objects move due to the forces that act on them.

B. Performance Standards

By the end of the Quarter, learners demonstrate a practical understanding of Newton's three laws of motion to describe relationships between variables and use these to explain everyday application of Newton's laws.

C. Learning Competency

CG Quarter 1, competencies 4 and 5:

4. explain that when any two objects interact, there are equal but opposite forces exerted between them, which is evident in many practical situations and applications;

5. observe and identify action-reaction pairs in everyday situations such as stepping off a boat, or a book on a table, and draw force diagrams to explain how the pairs affect the motion of objects.

By the end of the session, learners can:

1. state the third law and, given any force, name its partner in the form B pushes A;
2. state that the two forces of a pair act on two different objects, and use that to explain why they do not cancel;
3. distinguish an action–reaction pair from two balanced forces on one object; and
4. explain how two equal forces can produce unequal motion.

D. Key Vocabulary

The word reaction is the single worst piece of vocabulary in Key Stage 3 physics and it is not the class's fault. Say the warning in §D of the vocabulary out loud on Day 1 and repeat it whenever somebody says and then the reaction happens.

The class sentence, said in §D and printed on the wall chart: "Nothing cancels unless it acts on the same object."

E. Integration Across Learning Areas

II. MATERIALS

Needed — no cost. Two chairs that slide — the whole lesson turns on them. A clear space of about two metres at the front. Paper and pencil. Chalk and a board.

If the chairs do not slide or the floor is rough, two learners standing in socks on a smooth floor will do, or two learners each sitting on a folded sack. If neither is possible, two learners standing and pushing palm to palm, with the class watching their feet — both slide back a little, and the lighter one slides further.

Useful and not needed. A length of rope for the tug of war in §D. One balloon.

Not used: a force sensor, a dynamics trolley, or spring balances. Two chairs demonstrate the whole of the third law and every learner can see them from a desk. A force sensor gives a number that only the person holding it can read.

In addition. slides.html slides 1–13 · worksheet.html · game.html Rounds 1–2 · wall-chart.html

III. TEACHING AND LEARNING PROCEDURE

A. Activating Prior Knowledge · 6 minutes

Slide 2. Describe it and let the class picture it:

"You are in a small boat next to a pier. You step forward off the boat onto the pier — and the boat slides backwards away from you. Everybody here has either done that or watched somebody do it."

Ask: "Why does the boat move?"

Take three or four answers and write them on the board. Do not settle it. Most classes offer because you pushed it — which is right — and then cannot say why the push on the boat did not have an equal push back that stopped everything.

Slide 3. Write the law and leave it up for the whole week:

When any two objects interact, the forces they exert on each other are equal in size and opposite in direction.

Ask the class to read it aloud once. Then say: "Everybody in this room can say that sentence. By Friday, some of you will believe it."

B. Establishing Lesson Purpose · 3 minutes

Say: "Today is not about learning the law. You just read it. Today is about the one question that makes it believable — and it is a question that people with physics degrees still get wrong."

Slide 4. Put the objection up in the class's own words:

• If the forces are equal and opposite…
• …shouldn't they cancel out?
• …shouldn't nothing ever move?

Say: "That is correct reasoning. It is correct reasoning from a wrong premise, which is the most dangerous kind, and finding the wrong premise is the lesson."

C. Developing and Deepening Understanding · 26 minutes

C1 · The answer, in six words · 6 minutes

Slide 5. Write it big and say nothing for a moment:

They act on different objects.

Then unpack it, slowly:

• You push the boat backwards. That force acts on the boat.
• The boat pushes you forwards. That force acts on you.

Say: "Two forces. Two objects. Nothing cancels — because cancelling only happens when two forces act on the same thing. Adding up forces on different objects is like adding up your height and somebody else's weight. The answer means nothing."

Slide 6. Now fix the language, because the language is the method:

Ask: "What does the full sentence tell you that the short one does not?"

Who is pushing. What is being pushed. Which way. All three matter and the short version hides the first two, which is precisely where the misconception hides too.

From this minute on, every force said aloud in this class names two objects. Enforce it gently and without exception, including on yourself. It costs about four seconds a sentence and it does more for this topic than any explanation.

C2 · Finding the partner · 4 minutes

Slide 7. Give the algorithm:

1. Take any force: A pushes B.
2. Swap the two names: B pushes A.
3. Same size. Opposite direction.

Say: "That is the partner. It is always this, and it is never anything else. If you can write the force as a sentence, you can find its partner without knowing any physics at all."

Do four aloud, fast, with the class answering in chorus:

The mango is worth four extra seconds. Say: "Yes, really. The mango pulls the Earth up with exactly as much force as the Earth pulls the mango down. The Earth is simply so massive that nothing measurable happens to it." That is the second law arriving early, and it is about to be needed.

C3 · Two chairs · 9 minutes

Slide 8. Do it. Do not describe it.

Two learners sit on chairs facing each other, feet off the floor, close enough to touch palms. Tell the class: "Only one of them is going to push. Watch both chairs."

Before the push, ask: "Only one person is pushing. Who moves?" Take a show of hands. Most classes vote that only the pushed one moves. Write the count on the board — it is the number the teacher reflection asks for.

Now the push. Both chairs move apart, every time.

Ask: "One push. Why did two people move?" Let the class reach it: the pusher's hands were pushed back by the other person, with the same force, at the same moment. There was never one force. There were always two.

Slide 9. Now run it again with a visibly heavier learner and a visibly lighter one, and ask the class to watch which one travels further.

The lighter learner goes further and faster, every time. Ask: "The forces were equal. So how did they end up moving differently?"

Draw it out: the same force on a smaller mass produces a bigger acceleration. That is Week 2's competency 3 — the relationship among force, mass and acceleration — and this is the moment it earns its place.

Say the sentence that resolves the week:

"Equal forces do not mean equal motion. What the force does depends on what it is acting on."

C4 · The trap · 7 minutes

Slide 10. Set it up carefully, because most of the class is about to be wrong and it matters that they are wrong out loud:

"A book is resting on a table. Force one: the Earth pulls the book down — that is its weight. Force two: the table pushes the book up. They are equal. They are opposite."

Ask: "Are they an action–reaction pair?" Take a vote and write the count on the board.

Most classes vote yes, and they vote yes confidently.

Slide 11. Give the test, not the answer:

"Do not tell me equal and opposite. Tell me what object each force acts on."

Let the class see it. 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 stays still, and pair forces never cancel.

Then supply the real partners:

Say: "Four forces here, not two. Two of them are on the book and they balance. The other two are on the Earth and on the table, and you never see them because nothing visible happens to a planet or a table."

This is the hardest thing in the week and the curriculum guide names it in the competency itself — such as stepping off a boat, or a book on a table. The guide gave the easy example and the hard one in the same line, and the hard one is here.

D. Making Generalisations · 6 minutes

Slide 12. The tug of war. Say: "Team A pulls Team B. Team B pulls Team A. Those two forces are equal — always, whoever wins."

Ask: "Then what decides the winner?"

The friction between each team's feet and the ground. The forces on the rope are equal by the third law; the forces from the ground are not, and that is the whole contest. A heavier team wins more often because more weight means more friction available, not because they pull harder.

Slide 13. Let a balloon go, if there is one; describe it if not. The balloon pushes air out backwards; the air pushes the balloon forwards. Ask: "Would that work in space, with no air outside to push against?" Yes — the balloon pushes on its own escaping air, and needs nothing outside at all. That is a rocket.

Slide 15. Say the class sentence together, twice:

"Nothing cancels unless it acts on the same object."

Hand out worksheet.html.

IV. EVALUATING LEARNING

A. Assessment · 4 minutes

Exit ticket, three lines. Both objects must be named in every answer.

1. A person pushes a boat backwards. Write the partner force as a full sentence.
2. Two learners on chairs push off each other. One is heavier. Who feels the larger force, and who moves further?
3. A book rests on a table. Are its weight and the table's push an action–reaction pair? How do you know?

Item 3 is the one that reads. Item 1 is on the wall chart by Wednesday and item 2 was demonstrated in front of them.

B. Teacher Reflection

1. Record two numbers from the board: the §C3 vote (how many said only one person would move) and the §C4 vote (how many said the book's two forces were a pair). Those two counts are the lesson, and the second one is the one that should still be high — if it was low, the question was asked in a way that gave the answer away.
2. How many learners, unprompted, named both objects when they spoke in the second half of the lesson? That habit is the competency. Count roughly; the trend across the week is what matters.
3. Did anyone say and then the reaction happens? That is the English word interfering, and it is worth thirty seconds tomorrow rather than a correction today.

C. Enrichment and Remediation

Anticipated Misconceptions

If There Is No Electricity

Nothing here needs it. Two chairs, a rope, a balloon and a board are the whole apparatus, and §III carries everything that is on the slides.

If there are no chairs that slide, two learners in socks on a smooth floor works, or two learners standing palm to palm with the class watching their feet. The demonstration is not optional — it is the only part of this lesson that changes anybody's mind, and every substitute above shows the same thing.

Part of the E-turo MATATAG artifact set. Companion files: syllabus.md · lesson-plan-ilaw.md · assessment.md · offline-activities.md