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# Where does the energy go?

Year 9 Physics · Conservation of energy and efficiency

<!-- Speaker note: Students state the law of conservation of energy and use it to account for all the energy in a system, including energy transferred as heat and sound (AC9S9U05, node 1, a hinge node). About 25 minutes (Engage and Explore 8 min · Explain 10 min · Evaluate 3 min · Typical adds 3 min, Wider about 2 min). -->

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## Today

**We are learning to** energy is never made or destroyed: it only moves and changes type. This is the law of conservation of energy. When energy seems to disappear (a ball bounces lower, a car slows down), it has gone into heat and sound in the surroundings. The total is always the same.

- I can state the law of conservation of energy.
- I can account for all the energy in a system, including heat and sound.
- I can explain where energy goes when it seems to be lost.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea. Set the width with `?width=minimum`, `typical` (default) or `wider`. It builds on Year 8 energy (AC9S8U05) and comes before How much is useful? (efficiency). The hinge: students who think energy is "used up" or "disappears" can't account for every output in an efficiency calculation and read Sankey diagrams as loss. The prediction and the braking question target it. Note the everyday meaning of "conserve energy" (save it) differs from the law. Board notes: the coaster's total energy is set at the start and drawn as a dashed line; with friction (a steady force while moving) the heat and sound bar grows. On the high-friction track the car settles in a dip. Real coasters also lose energy to air resistance that grows with speed. Teacher: In your own words: three sets of bars that each add up to the total. -->

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## Why it matters

> Drop a ball and it bounces back up, but never quite as high. Each bounce is lower until it stops. Has its energy been used up?

<!-- Speaker note: In Year 8 you followed energy through systems and found by-products. This lesson turns that into a law. -->

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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 ball bounces lower each time until it stops. What happened to its energy?**

- It became heat and sound in the ball, the floor and the air
- It was used up
- It disappeared when the ball stopped
- It went back into the person who dropped it

<!-- Speaker note: What happens: It became heat and sound. Each bounce, some of the ball's energy is transformed into heat (the ball and floor warm very slightly) and sound (the bounce you hear). None of it disappears: it is spread into the surroundings. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: tally the predictions. 'Used up' and 'disappeared' are the hinge misconceptions. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-9%2Fconservation%2Fstep-2&code=widget%3A+energy%0Amode%3A+coaster%0Afriction%3A+toggle%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/physics-9/conservation/?full=step-2#step-2)

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: ask whether the total line ever moved, with or without friction. -->

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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 became heat and sound. Each bounce, some of the ball's energy is transformed into heat (the ball and floor warm very slightly) and sound (the bounce you hear). None of it disappears: it is spread into the surroundings.

<!-- 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>

## The law

**What does the law of conservation of energy say?**

- Energy is used up when things move
- You should save energy by switching things off
- Energy can't be made or destroyed, only transferred or transformed; the total stays the same

* Answer: Energy can't be made or destroyed, only transferred or transformed; the total stays the same

<!-- Speaker note: In any system, total energy in = total energy out. -->

<!-- Speaker note: Teacher: the law, the 150 kJ calculation and the braking question are the success criteria. -->

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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>

## Count every kJ

**A coaster car has 150 kJ in total. At one point it has 90 kJ of potential energy and 40 kJ of kinetic energy. How many kJ have become heat and sound?**

* Answer: 20

<!-- Speaker note: 150 − 90 − 40 = 20 kJ. Every kJ is accounted for. -->

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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>

## Braking

**People say a car 'loses' energy when it brakes. What really happens?**

- The energy is destroyed by the brakes
- Its kinetic energy becomes heat in the brakes, which spreads into the air
- The energy goes back into the fuel

* Answer: Its kinetic energy becomes heat in the brakes, which spreads into the air

<!-- Speaker note: 'Lost' means 'transferred somewhere we can't use it'. Feel a bike's brakes after a long downhill: they're hot. -->

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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 pendulum

**Why can a pendulum never swing higher than where it started?**

- It would need more energy than it had: energy can't be made
- Pendulums only swing lower
- Gravity gets stronger

* Answer: It would need more energy than it had: energy can't be made

<!-- Speaker note: With no extra energy put in, the most it can do is reach its starting height, and friction makes it a little lower each swing. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Elaborate: a lot of friction

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-9%2Fconservation%2Fstep-4&code=widget%3A+energy%0Amode%3A+coaster%0Afriction%3A+on%0Adrag%3A+1500%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/physics-9/conservation/?full=step-4#step-4)

<!-- Speaker note: What this step is for: A car that comes to rest shows the end point of every real system: all the energy spread out as heat. -->

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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>

## Where it went

**The car ended up stopped in a dip. Where is its starting energy now?**

- It is kinetic energy, waiting to be used
- It has all gone
- Most is heat and sound in the surroundings; the rest is potential energy where it rests

* Answer: Most is heat and sound in the surroundings; the rest is potential energy where it rests

<!-- Speaker note: The bars still add up to the total: heat and sound, plus the potential energy of its resting height. -->

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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>

## Elaborate: perpetual motion

**An inventor claims a wheel that keeps spinning forever and also powers a light. Why is this impossible?**

- It would spin too fast
- The light's energy and the friction heat must come from somewhere; with no energy in, the wheel must slow down
- Wheels can't power lights

* Answer: The light's energy and the friction heat must come from somewhere; with no energy in, the wheel must slow down

<!-- Speaker note: Energy out (light, heat) with nothing in would mean making energy. Every machine needs an input. What this step is for: Perpetual motion machines are a classic test of the law: any that claim to run forever while doing work break it. -->

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## Evaluate: look back

Write one sentence: "At first I thought… Now I think… because…"

**Revision summary**

- **Conservation of energy:** energy is never made or destroyed; it is only transferred or transformed. The total stays the same.
- Energy that seems "lost" or "used up" has been transferred to the surroundings, usually as heat and sound.
- Total energy in = total energy out, so you can account for every joule.

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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 skateboarder has 2000 J at the top of a ramp and 1700 J of kinetic energy at the bottom. Where are the other 300 J?**

* Transferred as heat and sound because of friction and air resistance. The total is still 2000 J.

<!-- 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: How much is useful? (efficiency, node 2). In class: drop balls of different kinds and measure bounce heights; feel a bike brake or sandpaper after rubbing. -->