▶ Force, Motion, and Energy · Quarter 1

Two Forces, Two Objects

The law is easy to say and almost impossible to believe. This is why.

Two chairs at the front, facing each other, before the class arrives. 1 min

Step off a boat

You step forward off a small boat onto the pier.
The boat slides backwards away from you.
Everybody has seen this, or seen it happen to somebody else.

Why does the boat move?

The guide names this example in competency 5. Take answers; do not settle it. 4 min

The law

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

Competency 4, close to word for word. Write it and leave it up all week. 3 min

The objection everybody has

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

So why does the boat move?

This objection is correct reasoning from a wrong premise. Say that out loud. 4 min

The answer, in six words

You push the boat backwards.
The boat pushes you forwards.
Two forces. Two objects. Nothing cancels, because cancelling only happens when two forces act on the same object.

They act on different objects.

This is the slide. Everything else in the week is a consequence of it. 5 min

Say it in full, every time

Not this

"There is a force of 20 newtons."

This

"The person pushes the boat with 20 newtons, backwards."

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

Who is pushing, what is being pushed, and in which direction. The naming convention is the method. 4 min

Finding the partner

Take any force: A pushes B.
Swap the two names: B pushes A.
Same size. Opposite direction. That is the partner.
It is always that simple, and it is never anything else.

Slide 7 is the algorithm. It goes on the wall chart. 4 min

Two chairs

Two learners sit on chairs facing each other, feet off the floor.
One of them pushes the other. Only one.
Both chairs move apart.

Only one person pushed. Why did both move?

Do it. Then ask before explaining. Slide 9 is the answer. 6 min

Now make them unequal

Run it again with a heavier learner and a lighter one.
The forces are still equal.
The lighter one moves further and faster.

How can equal forces produce unequal motion?

Different masses. Second law meets third law, which is why this week follows Week 2. 5 min

The trap

Force 1: the Earth pulls the book down — its weight.
Force 2: the table pushes the book up.
Equal. Opposite.

A book rests on a table.

Are they an action–reaction pair?

Take a vote. Most classes say yes. Write the count on the board. 4 min

No — and here is the test

Both act on the BOOK

Weight acts on the book. The table's push acts on the book. Same object — so this is a balanced pair, not an action–reaction pair.

The real partners

The Earth pulls the book down → the book pulls the Earth up.<br>The table pushes the book up → the book pushes the table down.

This is the guide's own second example, and it is the hardest thing in the week. 6 min

Tug of war

Team A pulls Team B. Team B pulls Team A.
Those two forces are equal, always, whoever wins.
So what decides the winner?

If not the pull on the rope, then what?

Friction with the ground. The rope forces are equal; the ground forces are not. 5 min

A balloon lets go

Air rushes out backwards.
The balloon pushes the air backwards.
The air pushes the balloon forwards.
That is a rocket, and it needs nothing to push against.

Would it work in space?

Yes. It pushes on its own exhaust, not on the air outside. Worth thirty seconds. 4 min

Four cases, one rule

Boat and person · chairs · book and table · tug of war
In every one, the two forces are equal and opposite.
In every one, they act on different objects.
In every one, the motion depends on mass and on the other forces present.

The wall chart's fourth panel is exactly this. 4 min

One sentence to leave with

Nothing cancels unless it acts on the same object.

Say it together. It is on the worksheet and on the wall chart. 2 min