An unbalanced force changes an object's motion, in the direction of the net force. Speeding up, slowing down, stopping, reversing and turning are all changes in motion, and every one needs an unbalanced force.
A curling stone glides 40 metres across the ice. Sweep in front of it and it slides further; let it hit another stone and it stops dead. Every time its motion changes, something has pushed or pulled it.
Last lesson: balanced forces keep the motion the same. This lesson: what an unbalanced force changes.
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.
Practise: A few on your own, to check you have it.
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 against the motion
The puck starts moving right at 4 m/s, and the ice has no friction. Press Play and watch the dots and the speed–time graph: a speed below zero means moving to the left. Try a bigger and a smaller push (Reset to start again).
Try it yourselfYour edits are saved in this browser.
With a push to the left, make the puck turn round and move to the left.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: any change in motion
In your book: draw force diagrams for two changes in motion, one where the speed changes and one where only the direction changes. Under each, name the unbalanced force.
Is the motion changing? (If it is, there must be an unbalanced force.)
A cyclist braking at the lights
A car going round a roundabout at a steady 30 km/hIts speed is steady but its direction keeps changing: that is a change in motion.
A lift rising at a steady speed
A ball thrown upwards, slowing as it risesWeight pulls it down the whole time: it slows, stops at the top and falls.
A book resting on a desk
A netball changing direction when it bounces off the backboard
A train sitting at a platform
PractiseA few on your own, to check you have it.
Mix it up
Elaborate TypicalUse the same idea in a new context.
Elaborate: from standing still
Try it yourselfYour edits are saved in this browser.
The box is at rest on a rough floor. Set the push so it starts moving to the right.
Use the board. This ticks when you get there.
The push is 7 N to the right and friction is 4 N to the left. What is the net force, in newtons?
Elaborate WiderUse the same idea in a new context.
Elaborate: longer pushes
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
An unbalanced force (a net force that isn't zero) changes the motion, in the direction of the net force.
A change in motion can be starting, speeding up, slowing down, stopping, reversing or changing direction. Every one needs an unbalanced force.
A force against the motion doesn't just stop an object: if it keeps acting, the object turns round and speeds up the other way.
Steady speed round a bend is still a change in motion (direction), so the forces are unbalanced.
For teachers
Students explain that unbalanced forces change motion (starting, speeding up, slowing down, stopping, reversing and changing direction) and represent each change with a force diagram (AC9S7U04, node 3). About 25 minutes.
I can explain that an unbalanced force changes an object's motion, in the direction of the net force.
I can name speed and direction as the two things that can change.
I can draw force diagrams for two different changes in motion.
Before class
A concept tutorial for introducing or revising the idea; it does not replace trolleys, force meters and ramps. Set the width with ?width=minimum, typical (default) or wider.
Not a hinge node, but its misconception is common: students apply "unbalanced force" to speeding up and miss slowing down, reversing and turning (and often think a backward force can only stop things). The Explore board shows one steady force slowing, stopping and reversing the puck. It builds on node 2 (Balanced), so set that first.
Board notes: the puck slides on frictionless ice and moves left and right only; turning and falling are covered by the sort and the round-the-bend question, not the board. Speeds below zero on the graph mean moving to the left. The Typical board has friction that matches the push up to 4 N while the box is still.
Running the lesson
Engage: tally the predictions. 'Slows down and stops, then stays still' is the one to watch.
Explore: ask what the graph does at the moment the speed crosses zero, and which way the net force points before and after.
Explain: the sort is the checkpoint (the roundabout and the thrown ball are the hard ones); the diagram question asks for a force diagram.
In your own words: two diagrams, one speed change and one direction change, are the node's success criteria.
The hard part (on purpose)
Seeing slowing, stopping, reversing and turning as changes that need an unbalanced force. When a student says the puck ran out of force, ask what it would do on ice with no push at all.
When students get stuck
A student says the puck turned round because it ran out of force. Ask: On ice with no push at all, what would the puck do? Fix: Keep sliding at the same speed. Moving needs no force; changing the motion does. The push changed it.
A student says the car on the roundabout has balanced forces because its speed is steady. Ask: Is it going in a straight line? Fix: No: its direction changes, so its motion changes, so the forces are unbalanced (the road pulls it towards the centre).
A student thinks a force only acts while the object moves in its direction. Ask: When did the speed start falling on the graph? Fix: The moment the push started: the force acts whichever way the puck is moving.
Stretch and support
Stretch: A ball is thrown straight up. At the very top it is still for an instant. Is the force on it balanced at that moment? (No: weight still acts, which is why it falls.)
Support: Give the sentence frame: 'The net force points … so the object … (speeds up, slows down, turns) …'.
Where it leads
Next: gravity, mass and weight (node 4). Node 5 (force, mass and the size of the change) is a class task: see the unit primer. In Year 10 this becomes Newton's first and second laws.
In class: a trolley pulled by a force meter, then released against a rubber band; a ball rolled across the floor and tapped sideways with a ruler (a change of direction only).
The spearthrower question (Wider) is general. For richer context, use local resources and community guidance about spearthrowers in your region.
What each step is for
Engage: the sliding puck: Students readily link a net force to speeding up, but expect a backward force only to stop things; the board shows the same force slowing, stopping and reversing the puck.
Explore: push against the motion: One steady force producing three different-looking changes (slowing, stopping, reversing) is the evidence that all of them are one effect: a change in motion towards the net force.
Explain: any change in motion: Widening 'change in motion' from speeding up to slowing, stopping, reversing and turning is the node's checkpoint: any change in motion needs an unbalanced force.
Mix it up: Whether forces are unbalanced is read off the arrows, so the force diagram of a resting object comes back first.
Elaborate: from standing still: Starting from rest with friction adds the third kind of change (starting) and makes students compare two arrows to find the net force before predicting.
Elaborate: longer pushes: A fair test and a First Peoples tool both turn on the same idea: the force, and how long it acts, decide how big the change in motion is.
Now you can
Explain that an unbalanced force changes an object's motion, in the direction of the net force.
Name speed and direction as the two things that can change.
Draw force diagrams for two different changes in motion.