Conservation of Energy · Grade 8 Science

Conservation of Energy

Learning goals

• I can state the law of conservation of energy.
• I can apply conservation of energy to describe energy changes in a system, including energy 'lost' to friction as thermal energy.

Readings for this unit

• A cool drink slowly warms

Energy can be transferred between parts of a system and across its boundary or redistributed among different stores. A complete energy account must include incoming and outgoing transfers. For an isolated system with no energy crossing the boundary, total energy remains unchanged. A system closed to matter can still exchange energy, so a sealed warm bottle can cool. Identify the boundary and conditions before choosing which energy total is expected to remain constant.

Yet everyday experience seems to argue the opposite. A swinging pendulum gradually loses height and stops. A ball rolling across the floor slows and comes to rest. Their mechanical energy — the combination of kinetic energy (motion) and potential energy (position) — clearly shrinks over time. If energy is really conserved, where does all that motion energy go?

The answer is that the energy doesn't disappear — it disperses. Friction between surfaces and air resistance quietly convert that mechanical energy into thermal energy (warming the surfaces a little) and sound. This spreading of energy into scattered, less useful forms is called energy dissipation. The total amount of energy is still exactly the same; it has just been scattered into forms too spread out and low-grade to easily gather back up and use.

A Newton's cradle shows the whole story in miniature. Lift one steel ball and let it swing into the row, and the energy travels almost perfectly from ball to ball, clicking out the far side and back again. 'Almost' is the key word — a sliver of energy escapes as the click you hear (sound) and as tiny amounts of heat at each collision. That's why the cradle gradually slows and finally stops: not because energy was destroyed, but because a little dissipates with every swing until none is left as visible motion.

This idea that energy is always conserved — merely changing form — is a powerful problem-solving tool. It lets you predict how fast a roller coaster will be moving at the bottom of a drop, because the energy it had at the top can't simply vanish; it must all reappear as motion, minus whatever friction dissipates along the way. Engineers lean on this same certainty when they design everything from bouncing safety nets to power stations: energy always has to be accounted for somewhere, so you can follow it, add it up, and trust that the books will balance.

Energy conservation is an accounting rule for a chosen system plus its transfers. In a closed accounting model, energy before plus energy entering equals energy after plus energy leaving. A useful output can fall while other stores increase.

Mechanical energy is the total of relevant kinetic and potential energies. It stays constant only when no energy is transferred by processes such as friction or external work. Total energy conservation applies more broadly, including thermal and chemical changes.

To analyze a roller coaster or falling object, start with height, speed and a reference level. List transfers that an ideal model ignores. Compare the prediction with measurements and explain why friction or air resistance may produce a lower speed without violating conservation.

Build your understanding

Where this fits

This unit helps answer the chapter question: Where does energy go, and how can we control its transfer? Keep a notebook table with the question, evidence, explanation and one unresolved issue. Revise a previous model only when you can name the new evidence or idea that changes it.

Use three teaching sessions of about 40–45 minutes: first observe the phenomenon, draft a model and study Parts 1–2; next explain Parts 3–4 with the figures and evidence; then work the application, examples, practice, model revision and exit check. Divide any difficult part into smaller steps and revisit after feedback. Investigations, review and quizzes have separate sessions.

Part 1 · Choose a complete energy boundary

Energy conservation compares all relevant stores and transfers within a chosen system and across its boundary. A closed matter system is not necessarily isolated from energy transfer. State whether energy can enter by work, heating, electrical processes, or radiation before expecting internal energy alone to remain unchanged.

Read the visual model

Does a sealed bottle necessarily conserve its own internal energy unchanged?

Visual reasoning — after your attempt

No. Sealing can prevent matter transfer while energy still crosses the boundary through heating or radiation. Its own energy store can decrease.

A sealed warm bottle can lose energy by heating its surroundings even though no water leaves. A matter boundary and an energy boundary answer different questions. Label both when evaluating a claim that a system is closed.

Matter is material that has mass and occupies space in this introductory account. Energy is a quantity describing motion, interactions and transfers in systems, rather than a material called heat. Radiation can transfer energy across a gap without carrying an ordinary matter sample across it. Matter and energy therefore require distinct accounts while being physically connected. Ordinary reactions retain the counted atoms while energy can transfer; advanced mass–energy relationships do not justify treating these classroom terms as interchangeable.

Pause and explain

Does a sealed bottle necessarily conserve its own internal energy unchanged?

Check your reasoning after answering

No. Sealing can prevent matter transfer while energy still crosses the boundary through heating or radiation. Its own energy store can decrease.

Part 2 · Count changes rather than confusing totals

For a given interval, initial stored energy plus incoming energy equals final stored energy plus outgoing energy in a complete account. Choose consistent units and avoid counting an internal transfer as a new external input. An energy decrease in one part should be matched by another store or outgoing transfer.

Read the visual model

How does a system gain stored energy while also sending energy outward?

Visual reasoning — after your attempt

If incoming energy exceeds outgoing energy, the net input increases the stored amount. Both transfers can occur in the same interval.

A system starts with 50 J, receives 20 J, and sends out 15 J. Its final stored energy is 55 J. The 20 J input is not a second initial store; it is energy crossing the boundary during the interval.

Pause and explain

How does a system gain stored energy while also sending energy outward?

Check your reasoning after answering

If incoming energy exceeds outgoing energy, the net input increases the stored amount. Both transfers can occur in the same interval.

Part 3 · Distinguish mechanical conservation from total conservation

Mechanical energy can shift to thermal stores and sound through friction or collision. Mechanical energy alone stays constant only under an appropriate model that excludes these transfers. Total energy accounting still applies. Using the word lost should mean lost from the useful or measured store, not destroyed.

Read the visual model

Mechanical energy is a subset of an energy account, not another name for every store. Check whether friction and other interactions transfer energy out of that subset before applying a mechanical-conservation shortcut.

How much mechanical energy has changed into the represented thermal contribution? Why is conservation of mechanical energy the wrong assumption for this case?

Visual reasoning — after your attempt

20 J. The account explicitly includes a thermal increase from interactions, so mechanical energy alone does not stay constant even though the full stated total does.

A pendulum swings to smaller heights because some mechanical energy transfers into other stores and surroundings. Its decreasing height challenges a no-loss mechanical model, not the conservation principle. Identify what the chosen measurements actually include.

Pause and explain

Why can mechanical energy decrease without violating energy conservation?

Check your reasoning after answering

Energy can transfer to thermal motion, sound, deformation, or surroundings. Mechanical energy is only part of the complete account.

Part 4 · Use a mismatch to guide a next measurement

A model and measured energy account may disagree because a transfer was omitted or estimates are uncertain. State the size of the discrepancy and propose a relevant measurement. Do not conceal the difference by adjusting numbers without evidence or label it proof of energy creation.

Read the visual model

Separate a numerical residual from an established mechanism. A revised explanation must connect the missing amount to evidence. State which additional records would support or challenge the proposed pathway.

What amount is unaccounted for, and name two specific measurements that could help resolve it?

Visual reasoning — after your attempt

100−60−25 = 15 J. Measure temperature/mass changes of relevant objects and other transfers or motion, while checking input measurement and timing/calibration.

A device has 80 J recorded input and 70 J measured output. The 10 J remainder may include heating or stored-energy changes. Measuring component temperature or additional outputs can test a proposal, though quantitative conversion needs material information.

Pause and explain

What is a useful response to an unexplained energy remainder?

Check your reasoning after answering

State the remainder, identify plausible missing transfers or stores, and propose measurements that could distinguish them. Keep uncertainty explicit.

Apply the whole unit · Complete two energy accounts

Illustrative account A: initial store 40 J, input 25 J, output 15 J. Account B: 50 J gravitational energy decreases while kinetic energy increases 35 J and measured thermal energy increases 10 J.

Your task

Find A’s final store and B’s unaccounted amount. Draw both boundaries and transfers, explain the difference between internal conversion and boundary transfer, and identify a useful follow-up measurement for B.

What your response should show

• Use the supplied observations and measurements accurately; include units where needed.
• Explain the science behind your answer, rather than giving only a number or label.
• Show a labeled model, calculation, or evidence comparison as requested.
• State an assumption or limitation and suggest a useful next measurement.

Response guidance — after your attempt

A ends with 40+25−15=50 J stored. B accounts for 45 J of the 50 J decrease, leaving 5 J unmeasured. Gravitational-to-kinetic conversion is internal for an object-Earth system; energy carried to outside surroundings crosses its boundary. Sound, other heating, or measurement error could explain the remainder, but evidence is needed to select a cause.

Scientific references and adapted teaching resources

• OpenSciEd 6.2 Thermal Energy — public unit overview
• OpenSciEd 8.3 Forces at a Distance — public unit overview
• OpenSciEd instructional model — storyline routines
• Creative Commons Attribution 4.0 — middle-school material license

Parent / teacher teaching notes

Use counters or bar charts to keep the accounting concrete. Ask whether each arrow crosses the chosen boundary. Accept explicit incompleteness when measurements do not support a full account. Adaptation credit: OpenSciEd middle-school teaching routines and public task structures by OpenSciEd, CC BY 4.0. This independent version adds original explanatory text, data and figures, terminology support and a home-study schedule. It does not reproduce the complete teacher editions or their classroom validation.

Law of Conservation of Energy

total energy before = total energy after

in a closed system, energy only changes form or location — it is never created or destroyed.

Energy Remaining After Losses

KE = PE − E lost

E lost is energy dissipated as thermal energy or sound, usually from friction or air resistance.

Worked examples

Avoid these mistakes

• Mechanical energy need not stay constant even when total energy is conserved.

Words to know

law of conservation of energy

Energy is conserved when stored changes and all transfers across the chosen boundary are counted. An isolated system exchanges neither matter nor energy; a system closed to matter can still exchange energy.

mechanical energy

The combined kinetic and potential energy of an object.

energy dissipation

Energy spreading out into less useful forms, like thermal energy or sound, often due to friction.

model

A representation that explains selected features and makes predictions; it leaves other details out.

evidence

Relevant observations or records used to support or challenge a claim.

system boundary

The chosen limit separating the objects being studied from their surroundings.

assumption

A condition taken as true for a model or calculation and needing to be checked against the case.

Two different meanings of 'conservation'

It's easy to mix up two very different ideas that share a word. The law of conservation of energy is a fundamental, always-true rule of physics: the total energy in a closed system never changes, no exceptions. 'Conserving energy' in everyday language — turning off lights, unplugging chargers — is a completely different idea: a recommendation to waste less useful energy as unwanted heat, not a law of physics. You can't 'break' the law of conservation of energy no matter how wastefully you act; you can only convert more of it into forms that are harder to use, like scattered heat.

The centuries-long hunt for a perpetual motion machine

For hundreds of years, inventors have proposed perpetual motion machines — devices that, once started, would supposedly run forever without any energy input, or even produce more energy than they consume. Every single design, when built and tested, has failed, always losing a little energy to friction, air resistance, or heat that it can't recapture. These repeated failures were part of the evidence that convinced scientists like Joule, Mayer, and Helmholtz in the 1840s that energy really is always conserved — and today, patent offices typically demand a working model before they'll even consider such a claim, precisely because the law has never once been broken.

Bungee jumping: three kinds of energy trading places at once

A roller coaster mostly trades two forms of energy back and forth, but a bungee jump involves three. At the top of the platform, gravitational potential energy is at its maximum. Partway down, the jumper is in free fall and kinetic energy peaks. At the very bottom of the stretch, the jumper has briefly stopped moving, but the stretched cord now holds maximum elastic potential energy — which then launches the jumper back upward, and the whole cycle repeats, losing a little energy to air resistance and the cord's internal friction on every bounce until the system settles.

Using the law as a detective's tool: finding hidden energy losses

Because energy can't vanish, engineers can use conservation of energy as an accounting shortcut: measure exactly how much energy goes into a system (like electricity into a motor) and how much comes out as useful work, and whatever energy is 'missing' from that count must have become waste heat or sound — even without measuring the heat directly. This exact detective technique, called an energy audit, is how engineers find hidden inefficiencies in everything from factory machines to home appliances, much like chemists use conservation of mass to calculate an unmeasured product just by subtraction.

In the real world

Hydroelectric dams convert the gravitational potential energy of stored water into electrical energy, all while obeying conservation of energy.

Exit check

How much mechanical energy is redistributed? Explain the relevant mechanism and one model limitation.