Newton's first law: an object at rest stays at rest, and a moving object keeps moving at a constant velocity, unless a net force acts. Moving things slow down because of friction, not because their push runs out.
Kick a ball on grass and it rolls to a stop. Slide a puck on ice and it goes much further. In space, a probe launched decades ago is still coasting out of the solar system with its engines off.
For 2000 years people believed, with Aristotle, that motion needs a force to keep it going. This lesson tests that idea.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: the sliding puck
ExploreTry it on the board and compare what happens with your prediction.
Explore: push, then let go
Press Play to push the puck. After a second or two, slide Push to 0 and turn Friction off. Watch the graph and the dots.
Try it yourselfYour edits are saved in this browser.
Get the puck moving, then turn the push to 0 and friction off, and watch it coast for a few seconds.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: the first law
In your book: someone says "the puck keeps going because the force of the hit is still in it". Write a reply using Newton's first law.
Elaborate TypicalUse the same idea in a new context.
Elaborate: friction is the hidden force
Try it yourselfYour edits are saved in this browser.
Let the moving puck slide with no push (friction on) until it stops.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: seatbelts
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
Newton's first law: an object stays at rest, or keeps moving at a constant velocity, unless a net force acts on it.
A moving object does not need a force to keep moving. A force is needed to change motion.
Everyday objects slow down because of friction and air resistance: hidden forces that are the net force.
Inertia is an object's resistance to changes in its motion; more mass, more inertia.
For teachers
Students state Newton's first law and explain why a moving object with no net force continues at constant velocity, confronting the Aristotelian idea that motion needs a continuous force (AC9S10U05, node 2, a hinge node). About 25 minutes.
I can explain why a moving object with no net force keeps a constant velocity.
I can identify friction as the hidden force that slows everyday objects.
Before class
A concept tutorial for introducing or revising the idea; it does not replace air tracks or a puck on a smooth floor. Set the width with ?width=minimum, typical (default) or wider.
The Aristotelian view survives exposure to the correct definition unless it is confronted. The Explore board is that confrontation: commit to a prediction, then watch the puck coast.
Board notes: motion is left and right only; friction is a fixed 2 N while sliding.
Running the lesson
Engage: ask students who chose 'slows down' to say where the force of the hit is stored. Keep their answers for Evaluate.
Explore: the check needs push 0 and friction off while moving. If a student turns friction off first, the puck speeds up: ask why.
Explain: the 'why does a ball stop' question is where the hidden force gets named.
In your own words: the reply to 'the force is still in it' is the evidence.
The hard part (on purpose)
Letting go of 'a moving thing needs a push to keep going'. When a student says the force ran out, ask what the graph did with push 0 and friction off.
When students get stuck
A student says the puck slows because the force runs out. Ask: On the board, with push 0 and friction off, did the graph slope down? Fix: No, it stayed flat. Nothing was stored; with no net force, nothing changes.
A student says inertia is a force. Ask: What pushes the puck along when it coasts? Fix: Nothing. Inertia is the tendency not to change motion, not a push.
A student confuses constant velocity with zero force. Ask: Could there be forces on a car moving at a constant 60 km/h? Fix: Yes, balanced ones. The net force is zero.
Stretch and support
Stretch: Voyager 1 is coasting out of the solar system with its engines off. Will it ever stop? What could change its motion?
Support: Give the frame: 'There is / isn't a net force, so the motion stays the same / changes, because …'.
Where it leads
Next: acceleration as a defined quantity (node 3), then F = ma (node 4).
In class: a puck or dry-ice block on a smooth bench; a coin on a card flicked off a cup (inertia).
What each step is for
Engage: the sliding puck: The Aristotelian answers (the force stays in the puck; it needs a push to keep going) are the target; committing first makes the coasting puck surprising.
Explore: push, then let go: Removing every force from a moving object and watching it coast confronts the 'motion needs force' belief directly.
Explain: the first law: Stating the law, then using it to explain the coasting puck and the everyday slowing down, replaces the Aristotelian model instead of adding to it.
Elaborate: friction is the hidden force: Running the same puck with friction on shows the everyday slowing as a net force effect, completing the replacement of the Aristotelian model.
Elaborate: seatbelts: The unrestrained passenger is the first law in a high-stakes setting: the car is stopped by a force; the passenger isn't.
Now you can
State Newton's first law.
Explain why a moving object with no net force keeps a constant velocity.
Identify friction as the hidden force that slows everyday objects.