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# What does an unbalanced force do?

Year 7 Physics · Forces and motion

<!-- Speaker note: 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 (Engage and Explore 8 min · Explain 10 min · Evaluate 3 min · Typical adds 4 min, Wider about 4 min). -->

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## Today

**We are learning to** 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.

- 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.

<!-- Speaker note: 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. Teacher: In your own words: two diagrams, one speed change and one direction change, are the node's success criteria. -->

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## Why it matters

> 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.

<!-- Speaker note: Last lesson: balanced forces keep the motion the same. This lesson: what an unbalanced force changes. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**A puck slides to the right across smooth ice. A steady push to the left starts acting on it and keeps acting. What do you predict happens?**

- It slows down and stops, then stays still
- It keeps sliding right at the same speed: it is already moving
- It slows down, stops, then moves off to the left, faster and faster
- It stops straight away

<!-- Speaker note: What happens: It slows down, stops, then moves off to the left, faster and faster. The net force points left the whole time, so the motion keeps changing towards the left: first slower, then reversed. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: tally the predictions. 'Slows down and stops, then stays still' is the one to watch. -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-7%2Funbalanced%2Fstep-2&code=widget%3A+forces%0Aobject%3A+Puck%0Amass%3A+2%0Aforce+P%3A+left%2C+2%2C+Push%2C+adjust+0-6%0Amotion%3A+on%0Aspeed%3A+4%0Atrack%3A+20%0Atime%3A+10%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/physics-7/unbalanced/?full=step-2#step-2)

<!-- Speaker note: 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). -->

<!-- Speaker note: Teacher: ask what the graph does at the moment the speed crosses zero, and which way the net force points before and after. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## What happened?

* It slows down, stops, then moves off to the left, faster and faster. The net force points left the whole time, so the motion keeps changing towards the left: first slower, then reversed.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Why it turned round

**On the board, why did the puck turn round?**

- The puck ran out of the force that was carrying it right
- The push only started working once the puck had stopped
- The push kept acting: the net force pointed left, so the puck slowed, stopped and then sped up to the left

* Answer: The push kept acting: the net force pointed left, so the puck slowed, stopped and then sped up to the left

<!-- Speaker note: An unbalanced force changes the motion in the direction of the net force for as long as it acts. Slowing down, stopping and reversing were all the same change: towards the left. -->

<!-- Speaker note: Teacher: the sort is the checkpoint (the roundabout and the thrown ball are the hard ones); the diagram question asks for a force diagram. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Changing or not

**Is the motion changing? (If it is, there must be an unbalanced force.)**

Groups: Changing: unbalanced force · Not changing: balanced forces

A cyclist braking at the lights · A car going round a roundabout at a steady 30 km/h · A lift rising at a steady speed · A ball thrown upwards, slowing as it rises · A book resting on a desk · A netball changing direction when it bounces off the backboard · A train sitting at a platform

* Answer: A cyclist braking at the lights → Changing: unbalanced force; A car going round a roundabout at a steady 30 km/h → Changing: unbalanced force; A lift rising at a steady speed → Not changing: balanced forces; A ball thrown upwards, slowing as it rises → Changing: unbalanced force; A book resting on a desk → Not changing: balanced forces; A netball changing direction when it bounces off the backboard → Changing: unbalanced force; A train sitting at a platform → Not changing: balanced forces

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Round the bend

**A car drives round a bend at a steady 40 km/h. Are the forces on it balanced?**

- No: its direction is changing, so there is a net force towards the inside of the bend
- Yes: its speed isn't changing
- There are no forces on it

* Answer: No: its direction is changing, so there is a net force towards the inside of the bend

<!-- Speaker note: A change of direction is a change in motion, so it needs an unbalanced force. Here the tyres' grip on the road pulls the car towards the inside of the bend. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Which diagram?

**A cyclist rides to the right and is slowing down. Which force diagram fits?**

- Forward push 70 N to the right, air resistance and friction 40 N to the left
- Forward push 40 N to the right, air resistance and friction 70 N to the left
- No forward push, and no other forces
- Forward push 50 N to the right, air resistance and friction 50 N to the left

* Answer: Forward push 40 N to the right, air resistance and friction 70 N to the left

<!-- Speaker note: Slowing down to the right means a net force to the left: the backward arrow is longer (70 N − 40 N = 30 N to the left). -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Mix it up

**Looking back at force arrows: a box rests on a table. Which arrows go in its force diagram?**

- Weight down and support up, the same length
- Weight and support, both pointing down
- Only weight, pointing down
- Weight down, longer than the support up

* Answer: Weight down and support up, the same length

<!-- Speaker note: Equal forces get equal arrows. Weight points down and support points up, so the arrows balance. What this step is for: Whether forces are unbalanced is read off the arrows, so the force diagram of a resting object comes back first. -->

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## Elaborate: from standing still

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-7%2Funbalanced%2Fstep-5&code=widget%3A+forces%0Aobject%3A+Box%0Amass%3A+2%0Aforce+P%3A+right%2C+2%2C+Push%2C+adjust+0-12%0Afriction%3A+4%0Amotion%3A+on%0Aspeed%3A+0%0Atrack%3A+20%0Atime%3A+6%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/physics-7/unbalanced/?full=step-5#step-5)

<!-- Speaker note: What this step is for: 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. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## The net force

**The push is 7 N to the right and friction is 4 N to the left. What is the net force, in newtons?**

* Answer: 3

<!-- Speaker note: 7 N − 4 N = 3 N to the right: unbalanced, so the box speeds up to the right. With a push of 4 N or less, friction matches it and the box stays still. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## A fair test

**You want to test whether a bigger push changes a trolley's speed more quickly. Which plan is a fair test?**

- Use a heavier trolley for the bigger push
- Use the same trolley on the same surface, and change only the size of the push
- Push one trolley on carpet and the other on tiles

* Answer: Use the same trolley on the same surface, and change only the size of the push

<!-- Speaker note: Change one variable (the push), measure one (how quickly the speed changes) and keep the rest the same (trolley, surface). -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## The spearthrower

**A spearthrower lets the thrower keep pushing on the spear over a longer arc. Why does the spear leave faster?**

- The spearthrower makes the spear lighter
- The unbalanced force acts on the spear for longer, so its speed keeps increasing for longer
- The spearthrower stores the spear's speed and releases it

* Answer: The unbalanced force acts on the spear for longer, so its speed keeps increasing for longer

<!-- Speaker note: The longer an unbalanced force acts, the more the motion changes. Spearthrowers, used by First Peoples across much of mainland Australia, extend the throwing arm so the push lasts longer. -->

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## 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.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**A netball flies towards the ring and bounces back off the backboard. Give the change in motion and the force that caused it.**

* It reversed direction (and slowed, then sped up the other way). The backboard pushed it back: an unbalanced force away from the board.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this 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. -->