▶ Force, Motion, and Energy

Nothing Cancels Unless It Acts on the Same Object

Science 9Term 1 · Week 3

1 Say it in full

Never <i>"a force of 20 N"</i>.
Always <b>"A pushes B, 20 N, to the right"</b>.
Who pushes · what is pushed · which way.
The naming is the method, not the manners.

"Who is pushing what?"

2 Find the partner

Take the force: <b>A pushes B</b>.
Swap the names: <b>B pushes A</b>.
Same size. Opposite direction.
It is always this, and never anything else.

"Swap the two names."

3 The two-object test

A pair acts on <b>two different objects</b>.
Two forces on <b>one</b> object is a <b>balanced pair</b>, not a partner pair.
Pair forces <b>never cancel</b> — they are not on the same thing.
<b>Equal and opposite is not enough.</b>

"Which object does each one act on?"

4 Where it looks broken

<b>Boat:</b> both move, different masses.
<b>Book:</b> weight and table push are NOT a pair.
<b>Tug of war:</b> rope forces equal; <b>friction</b> decides.
<b>Rocket:</b> pushes its own exhaust — works in space.

"Equal forces, unequal motion."