Potential and Kinetic Energy, Grade 7: Transfer, Conservation and Waves
"The wave makes the boat bob up and down. The wave makes the boat move in the direction of the wave."
Those two sentences sit next to each other and the second one contradicts the first. A water wave carries energy, not water. A floating object bobs in place and stays where it is.
And then the topic sets an investigation that disproves its own text. Learners are asked to work out how a doll ended up at the edge of a pool. Make waves without making a current and the doll bobs and stays put. That is the whole lesson about waves, handed over for free.
Waves move energy, not matter
The test a class can run in a basin in two minutes.
Float a cork at one end of a tray of water. Tap the surface rhythmically with a finger at the other end, making small waves, and keep your finger in one place.
The cork bobs. It does not travel.
Now stir the water instead, in one direction. The cork crosses the tray. That is a current, not a wave, and it is moving water rather than energy through water.
The case study in this topic is built on the wrong half of it. Someone beats and stirs the water, and a floating doll reaches the far side. The stirring is what moved it. What crossed the pool was a bulk current of water, not a wave carrying the doll along.
Why it matters beyond this topic: every wave a learner meets later behaves this way. Sound moves energy through air without the air travelling across the room. Light moves energy with nothing travelling at all. Getting it wrong here costs marks in Grade 10 and again in Grade 12.
One picture to ignore while you are at it: a figure captioned about sea waves transferring energy to a boat shows a cargo ship under its own power, which is the one thing in the photograph that is not being moved by waves.
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 7 |
| Term | 3 |
| Strand | Energy and Change |
| Topic | Potential and kinetic energy |
| Status | Class activities. Nothing here is formally assessed |
| Marks | The topic revision is 25. Our worksheet is 40 |
Safety: two activities we would not run as written
These are the two most dangerous instructions in the Grade 7 year, and neither carries an adequate warning.
1. The sealed tin heated over a flame
You will see a method along these lines: take a tin with a tightly fitting lid, fill it with warm water, press the lid on and make sure it fits tightly, and light a burner underneath it.
That is a sealed, full container of water being heated. It is a pressure vessel.
Some versions tell you to punch a small hole in the side just below the lid. But the tin is to be filled, so that hole sits under water, and as the contents expand it will jet boiling water sideways at whoever is nearest. If the hole is blocked, or never made, there is nowhere for the pressure to go.
| Our version | Why |
|---|---|
| Never seal a container you are heating | There is no safe version of a closed vessel over a flame in a classroom |
| Fill it no more than one third | Leaves room for the water to expand and for steam to collect |
| Keep any vent well above the water line | So steam escapes, not boiling water |
| Better: use an open beaker | The lesson is that heat becomes movement. An open beaker with a thermometer shows the same energy transfer and cannot build pressure |
2. The rubber band fired across the room
You will read that if you stretch a rubber band and release it, it flies across the classroom, and that stretching it further makes it fly further. There is no eye-safety note anywhere in the topic, and a bent ruler flicking a paper pellet, plus a catapult photograph, follow within a few pages.
The physics is right and the delivery is wrong. Keep the physics and change the delivery.
Stretch the band and let it pull something instead of launching it. Hook it round a wooden block on a smooth desk, pull it back a measured distance, release, and measure how far the block slides.
Stored energy goes in, work comes out, and the measurement is better than the original. Pull it back twice as far and the block goes further, which is exactly the point the original was making, with nobody aiming anything at a face.
The three kinds of potential energy
| Kind | Stored because | Everyday example |
|---|---|---|
| Gravitational | Something has been lifted | A book on a shelf, water in a reservoir above a town |
| Elastic | Something has been stretched or squashed | A drawn bow, a wound spring, a stretched rubber band |
| Chemical | Energy is stored in the bonds between atoms | A battery, food, petrol, firewood |
Kinetic energy is the other half: the energy a thing has because it is moving. Heavier and faster both mean more of it, and speed matters far more than mass, which is why a small stone thrown hard does more damage than a big one dropped.
Energy is measured in joules. South African food labels give kilojoules per 100 g, so a learner can read real numbers off a packet: chocolate around 2 200 kJ, breakfast cereal around 1 300, roasted peanuts around 2 500, and pure sugar around 1 700. Peanuts beating chocolate is the result nobody expects, and it is because fat carries more than twice the energy of sugar per gram.
One caption that says the opposite of the lesson
You will see a figure of something being hauled up on a rope, captioned to the effect that the object's potential energy "has not been created by anything or anyone".
Somebody did put that energy there. The person pulling the rope converted chemical energy from their own muscles into gravitational potential energy in the load. That is the whole point of the picture.
The caption is reaching for "energy cannot be created from nothing", which is correct, and lands on a sentence that denies the transfer the diagram is illustrating. Energy was not created. It was moved, and the picture shows who moved it.
While you are in the key words, one more: thermal energy defined as "energy produced by heat" is circular. Thermal energy is the internal energy a substance already has, mostly the kinetic energy of its particles. Heat is the transfer of it. Heat is what happens; thermal energy is what you have.
The comeback can, and why it does not come back
A tin, a rubber band stretched down its length, a weight hanging on the middle of the band, and the lid back on. Roll it away and it rolls back to you.
The method as written says to give the can "a slight push". It will not come back.
The can returns because the hanging weight stays at the bottom while the tin rotates around it, so the band winds up and stores elastic potential energy. A slight push stores almost nothing.
Roll it a good three or four metres so the band winds several turns. And put the lid back on after threading the band, which the method never actually says.
Energy flow diagrams, which are genuinely well taught
Input, process, output. The method is sound and worth using on everything in the topic.
| System | Input | Process | Useful output |
|---|---|---|---|
| Torch | Chemical, in the cell | Electrical | Light |
| Catapult | Chemical, in your arm | Elastic | Kinetic |
| Hydroelectric scheme | Gravitational, in the dam | Kinetic in the turbine | Electrical |
| You, running | Chemical, in food | Chemical in muscle | Kinetic |
One picture to ignore: a figure captioned as a water wheel that shows a waterfall over rocks with no wheel in it. The method is fine; the photograph is of the input only.
If it does not work
| Problem | Cause | Fix |
|---|---|---|
| The floating object crosses the tray | You made a current, not a wave. Your finger moved sideways | Tap in one spot. The cork should bob and stay |
| The class concludes waves carry objects along | The printed sentence | That is the finding to report. Waves carry energy; the current carried the doll |
| The comeback can does not come back | It was given a slight push | Roll it three or four metres so the band winds up. Check the lid is on |
| The weight inside spins with the can | It is too light, or caught on the band | Use a heavier weight hanging free at the middle |
| The rubber band snaps | Over-stretched, or old and perished | Fresh bands, and pull back a measured distance rather than as far as it will go |
| Food label energies do not compare | Different serving sizes | Always use the per 100 g column. That is what it is there for |
How the 40 marks are made up
| Part | Marks |
|---|---|
| The three kinds of potential energy | 8 |
| Kinetic energy, joules and reading a food label | 8 |
| Energy transfer and conservation | 8 |
| The wave investigation | 10 |
| Safety and experimental design | 6 |
Nothing in this topic is formally assessed. The topic revision is 25 marks and it adds up correctly. Our worksheet is 40 and the split above is ours.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks, including the wave investigation and the sealed tin to redesign
- Marking memorandum, with what to accept and the five answers that look right and score nothing
Related practicals
- Conservation of Energy with a Simple Pendulum, Grade 10. The senior version, where the energy is measured rather than described
- The Particle Model of Matter, Grade 8. The hot and cold diffusion comparison, done properly and timed
- Insulation and Energy Saving, Grade 7. Where the wasted energy goes, and the assessed Term 3 project
- Transverse Pulses and Waves, Grade 10. Five experiments on one spring, and the proof that a pulse moves without the spring moving
What you need to run it
| Item | Price | What it is for |
|---|---|---|
| Beaker, borosilicate | R7 | The safe replacement for the sealed tin, and the diffusion comparison |
| Thermometer, red spirit | R35 | Turns the heat-to-movement demonstration into a measurement |
| AA battery holder with leads | R6,50 | The electrical system: chemical energy in, light out |
| Bulb, MES E10 | R20,64 | The output end of that system |
| Crocodile leads, pack of 10 | R58 | Five circuits from one pack |
| Safety goggles | R30 | Anything with hot water or a stretched band near a face |
About R90 of circuit parts per group, most of which a school already owns, and a beaker instead of a sealed tin.
What we do not sell, and the topic does not need from us: rubber bands, tins, string, corks, a basin, food packets, modelling clay. The valuable thing in this topic is a redesign, not a purchase: an open beaker instead of a closed tin, and a rubber band that pulls instead of one that flies.
Glassware is in laboratory glassware and the circuit parts are in electricity.