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.
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?
In Year 8 you followed energy through systems and found by-products. This lesson turns that into a law.
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 bouncing ball
ExploreTry it on the board and compare what happens with your prediction.
Explore: count it all
Try it yourselfYour edits are saved in this browser.
Run the coaster with friction off. Does the total change?
Use the board. This ticks when you get there.
Now turn friction on and run it. Where does energy go, and does the total change?
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: the law
In your book: draw the three bars for the coaster at the start, at the bottom of the first dip and at the end (with friction). Show that each set adds up to the same total.
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?
Elaborate TypicalUse the same idea in a new context.
Elaborate: a lot of friction
Try it yourselfYour edits are saved in this browser.
This track has much more friction. Run the car until it stops.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: perpetual motion
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
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.
For teachers
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.
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.
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.
Running the lesson
Engage: tally the predictions. 'Used up' and 'disappeared' are the hinge misconceptions.
Explore: ask whether the total line ever moved, with or without friction.
Explain: the law, the 150 kJ calculation and the braking question are the success criteria.
In your own words: three sets of bars that each add up to the total.
The hard part (on purpose)
Replacing 'used up'. When a student says energy was used up, ask 'used up into what?' and wait for them to name heat or sound.
When students get stuck
A student says the energy was used up. Ask: Into what? Was anything warmer or louder afterwards? Fix: It became heat and sound. It's still there, spread into the surroundings.
A student thinks friction destroys energy. Ask: What did the red bar do when friction was on? Fix: It grew: friction transforms kinetic energy into heat. The total stayed the same.
A student confuses the law with saving energy at home. Ask: If you leave a light on, is energy destroyed? Fix: No, it's transformed into light and heat. 'Saving energy' means using less of a useful supply, not stopping it being destroyed.
Stretch and support
Stretch: A ball dropped from 2 m bounces to 1.5 m. What fraction of its energy became heat and sound on the bounce? (A quarter: it has three quarters of its starting potential energy at the top of the bounce.)
Support: Give the equation as a frame: 'Total = potential + kinetic + heat and sound = … + … + … '.
Where it 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.
What each step is for
Engage: the bouncing ball: 'Used up' and 'disappears' are the hinge misconceptions; the bouncing ball forces a choice between losing energy and moving it.
Explore: count it all: Running with and without friction, and seeing the total line never move, makes 'the total stays the same' an observation before it is a law.
Explain: the law: Stating the law, accounting for every joule in a calculation, and re-reading 'lost' energy as 'transferred to the surroundings' resolves the hinge.
Elaborate: a lot of friction: A car that comes to rest shows the end point of every real system: all the energy spread out as heat.
Elaborate: perpetual motion: Perpetual motion machines are a classic test of the law: any that claim to run forever while doing work break it.
Now you can
State the law of conservation of energy.
Account for all the energy in a system, including heat and sound.
Explain where energy goes when it seems to be lost.