GCSE · Physics · Edexcel · Spec 1PH0

Analyse energy stores in system changes

A spring flicks an object into the air, and it ends up with less energy than the spring gave up. Where did the rest go? Time to track every joule.

One energy journey, stage by stage

Stage 1 of 4: Spring stretched. paused

Spring stretched · 1/4Scrub along: which store is emptying, which is filling?

Elastic store: the energy is stored here, ready to be transferred.

Press play or drag along the journey. At each stage, name the store that empties and the store that fills.

Where did it go?

Which idea sounds like yours?

A stretched spring launches an object upwards. By the end, the object's gravitational store holds less energy than the elastic store of the spring gave up.

Where has the difference gone? Pick the idea closest to what you honestly think.

Energy gets used up when it does a job. The spring did its job, so that energy is gone.Air resistance destroys some of the energy, so there is less in total at the end.The gravitational store gains exactly the energy the elastic store lost. Energy in equals energy out.Nothing has vanished. Some energy was dissipated into the surroundings, so the gravitational store ends up with less.

How sure are you?

I'm sureI think soI'm guessing

Energy gets used up when it does a job. The spring did its job, so that energy is gone.

This is a common misconception

That feels right, because the spring clearly has less than before. But energy is never used up. It is transferred from one store to another. When the spring does its job, its elastic store loses energy and the object's kinetic store gains it. The energy has moved, not vanished.

Air resistance destroys some of the energy, so there is less in total at the end.

This is a common misconception

Tempting, because the object really does end up with less in the store we wanted. But energy cannot be destroyed, so the total at the end equals the total at the start. Air resistance dissipates energy: it transfers it into the surroundings by warming them. That energy is still there, in the thermal store of the surroundings.

The gravitational store gains exactly the energy the elastic store lost. Energy in equals energy out.

This is a common misconception

That is true only in an ideal situation, where no energy is dissipated. In real transfers some energy usually is dissipated, so the energy in the intended store is less than the energy that left the original store.

Nothing has vanished. Some energy was dissipated into the surroundings, so the gravitational store ends up with less.

This matches the evidence

Yes. Friction and air resistance warm the surroundings, so the missing energy is in the thermal store of the surroundings. The total at the end still equals the total at the start. Dissipation is why the intended store gets less.

Energy is never used up or destroyed. When some seems to go missing, it has been dissipated into the thermal store of the surroundings, and that is exactly why the intended store ends up with less.

Work it out

A student pushes a loaded trolley with a steady force of 250 N, and it moves 12 m in the direction of the force. How much energy is transferred (the work done)? Give the answer in joules, then convert it to kilojoules (1 kJ = 1000 J).

Finding work done (J)

W = F d

force · F

distance · d

Answer

W = 250 × 12

= 3000 J

Predict, then check

Don't scroll yet. Pick what you think, then reveal.

A spring is stretched until it stores some energy. You stretch the same spring twice as far. What happens to the energy in its elastic store?

Check someone else's working

Where does this answer go wrong?

A trolley of mass 5 kg moves at 4 m/s. Calculate its kinetic energy.

A student's answer — which line goes wrong?

Kinetic energy = ½ × mass × (speed)²
Kinetic energy = (½ × 5 × 4)²
Kinetic energy = 10²
Kinetic energy = 100 J

Your turn to fill the gaps

Energy in the final store

A toy cart of mass 0.35 kg rolls down a smooth ramp and drops through a height of 12 m. Assume no energy is dissipated and use g = 10 N/kg. How much energy is in the cart's kinetic store at the bottom?

Stores first: the cart's gravitational store loses energy and its kinetic store gains it.
missing step

Which line is step 2?

Principle: with no energy dissipated, change in GPE = change in KE.Principle: the change in KE is smaller than the change in GPE, because falling dissipates energy.Principle: the change in KE equals the change in height.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Follow the energy: one store empties, another fills, and the total never changes.

What you need to know

Energy can't be created or destroyed. In any system change, some stores lose energy and others gain it, so the total stays equal.
Stores to know: elastic (springs), gravitational (raised objects, GPE), kinetic (moving objects, KE) and the thermal store of the surroundings.
In real transfers some energy is usually dissipated: friction and air resistance warm the surroundings, so the intended store gets less.
Have a goMo rolls down a ramp, reaches the bottom and announces: 'The energy the kinetic store is missing? Used up. Gone. Poof.' Where did it actually go?
Into the thermal store of the surroundings. Friction and air resistance warmed them.
Energy is never used up. Dissipated energy has moved into the surroundings, which is why the intended store ends up with less.
With no dissipation, the energy one store loses another gains, so change in GPE = change in KE.
Change in GPE = mass × gravitational field strength × change in height. The examples here use g = 10 N/kg.
Have a goA 3 kg book is lifted 2 m. Using g = 10 N/kg, what is the change in its gravitational potential energy?
60 J
Multiply mass × g × change in height: 3 × 10 × 2 = 60 J. Doing 3 × 2 = 6 leaves out g, and that is the tempting slip.
Kinetic energy = ½ × mass × (speed)². Only the speed is squared, not the whole product.
Work done = force × distance moved in the direction of the force, in joules (J). 1 kJ = 1000 J.
Have a goA lift does 12 kJ of work. How many joules is that?
12 000 J
1 kJ is 1000 J, so going from kJ to J means multiplying by 1000. Dividing instead is the tempting slip.
A stretched spring's elastic energy = ½ × spring constant × (extension)², with the spring constant in N/m.
Doubling the extension gives four times the energy. Doubling the spring constant doubles it.
Method: state the principle, write the equation, substitute with units, calculate, then name the energy in the final store.

The big picture

Energy is never created or destroyed. In a system change it moves from some stores to others, so analysing a change means spotting which stores lose energy and which gain it. In real transfers some energy is usually dissipated into the thermal store of the surroundings, so the intended store ends up with less. Four equations put numbers on the stores, and a five-step method keeps the working tidy.

Key points

1Analysing a system change: which stores lose energy, which gain it, and where does the rest go?

2Energy is conserved. Dissipated energy is not destroyed: it warms the surroundings, so it sits in their thermal store.

3Change in GPE = change in KE only when nothing is dissipated.

4Four equations: GPE, KE (speed squared only), work done (force × distance) and elastic energy (extension squared).

5Method: principle, equation, substitute with units, calculate, name the final store.

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Worked example

Problem

A 2.0 kg ball is dropped from a height of 6.5 m (g = 10 N/kg). Just before it lands, its kinetic store holds 118 J. How much energy was dissipated, and where did it go?

⚠ Watch out

Believing friction destroys energy. It dissipates it into the thermal store of the surroundings, so the total is conserved and the intended store just ends up with less. Also: in KE, square only the speed.

🧠

Memory hook

Energy is a bank balance, not a bonfire: it moves between accounts and never burns away. Dissipation is the leak into the surroundings' account, which is why your target account ends up lower.

✓

Check yourself

A compressed spring pushes a trolley up a slope. Name the store losing and the store gaining at each stage. Why does the gravitational store end up with less?

Flashcards

(13)

What is conservation of energy?

Energy cannot be created or destroyed. In any process it moves from one store to another, so the total at the end equals the total at the start.

What does it mean to analyse a system change?

Work out which energy stores gain energy and which lose it.

Name four energy stores used in this topic.

Elastic (stretched or compressed spring), gravitational (raised object), kinetic (moving object) and the thermal store of the surroundings.

What does 'dissipated' mean for energy?

Friction and air resistance transfer energy into the surroundings by warming them. The energy is spread out, not destroyed.

Why does the intended store usually end up with less energy in a real transfer?

Some energy is dissipated into the surroundings, so less reaches the intended store than left the original store.

When does the change in GPE equal the change in KE?

In an ideal situation, when no energy is dissipated.

How do you find a change in gravitational potential energy?

mass × gravitational field strength × change in height. The examples use g = 10 N/kg.

In kinetic energy = ½ × mass × (speed)², what gets squared?

Only the speed, not the whole product.

What is the equation for work done, and its unit?

Work done = force × distance moved in the direction of the force, measured in joules (J).

How many joules are in 1 kJ?

1000 J.

How do you find the energy in a stretched spring's elastic store?

½ × spring constant × (extension)². The spring constant is in N/m.

What happens to elastic energy if you double the extension? And the spring constant?

Doubling the extension gives four times the energy. Doubling the spring constant doubles it.

What are the steps for analysing an energy transfer calculation?

State the principle, write the equation, substitute with units, calculate, then state the energy in the final store with sensible significant figures.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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One energy journey, stage by stage

Which idea sounds like yours?

Where does this answer go wrong?

Energy in the final store

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