▶ Science 9 · Term 1 · Week 3

Ten Minutes at Home: Why the Boat Moves

Nothing to buy. Ten minutes with two chairs, or a wall and a smooth floor. Your child is learning Newton's third law, which is four words long and which almost nobody believes the first time. This activity gives them the one experience that makes it believable.

The activity: push off

minutes: 10 Materials: two chairs that slide, or a smooth floor and a wall. Socks help.

  1. Sit facing each other on two chairs, feet off the floor, close enough to touch hands.
  2. Ask: <q>If only I push, who moves?</q> Let them answer before you try it.
  3. Now push — only you. Both chairs move apart.
  4. Ask: <q>I only pushed once. Why did we both move?</q>
  5. Do it again with the other person pushing. The same thing happens.
  6. Ask the good one: <q>Did one of us move further? Why do you think that was?</q>
  7. If you weigh more, you moved less. Say: <q>Same push, different weight, different distance.</q>

Three things to avoid

What usually happensInstead
Saying <q>the forces cancel out</q>.They do not, and this is the exact idea the week is fighting. The two forces act on two different people, and forces only cancel when they act on the same thing.
Saying the heavier person <q>pushed harder</q>.The pushes are exactly equal, always. What differs is what the same push does to a heavier body — it moves it less. Your child is being assessed on precisely this distinction.
Answering the questions yourself when they hesitate.Count silently to ten first. The whole value of this topic is the moment the learner works out that two things moved from one push, and handing it over removes it.

Three signs it is working

If you only do one part, do step 4 — <b>I only pushed once, why did we both move?</b> Everything else in the topic is a consequence of the answer, and it is the question that most learners are never actually asked.