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Energy as stores belonging to things, and a transfer as one going down while another goes up.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
You will describe any everyday event as an energy transfer: which store went down, which store went up, and by what path — instead of naming a type of energy and stopping.
You can name both ends of a force and say which way it pushes. Energy works the same way: it is about two things at once, and about a change, not about a label sitting on one object. If that sounds familiar, it should.
You also know that things speed up, slow down, heat up and cool down. Every one of those is a change, and this lesson gives you a way to describe exactly what changed and where it went.
| Term | What it means |
|---|---|
| Energy | What is counted when something moves, heats up, is lifted or is stretched. |
| Store | Energy a particular thing has at a particular moment: the ball's raised store. |
| Transfer | One store going down while another goes up by the same amount. |
| Path | How a transfer gets across: by a force, by heating, by light or by an electric current. |
| Moving store | Energy something has because it is moving. |
| Raised store | Energy something has because it has been lifted up. |
| Stretched store | Energy in a stretched or squashed spring, band or bow. |
| Chemical store | Energy in food, fuel and batteries. |
| Thermal store | Energy in how warm something is. |
Energy is counted in stores, and every store belongs to something. The ball has a moving store and a raised store. The battery has a chemical store. The room has a thermal store.
Nothing ever happens to one store on its own. The only event there is:
> one store goes down and another goes up, by the same amount.
Drop a brick: its raised store goes down and its moving store goes up. Fire a slingshot: the band's stretched store goes down and the pellet's moving store goes up. Rub your hands: their moving store goes down and your skin's thermal store goes up.
And each one has a path — how it got across. By a force, by heating, by light, or by an electric current.
In this course, stores are counted in plain units. If $30$ units leave one store, $30$ units arrive in another.
Another way: picture
Two buckets joined by a pipe. Tip one and the other fills by exactly as much; the water is never anywhere but in a bucket or in the pipe. What the picture gets right is that the two levels always move together, and in opposite directions.
Another way: steps
Compare two ways of describing the same event.
| What is written | What it tells you |
|---|---|
| the arrow has kinetic energy | nothing that could be checked |
| the bow's stretched store went down and the arrow's moving store went up | which two things changed, and which way |
The first sentence cannot be wrong, because it does not say anything has happened. The second could be wrong — and that is what makes it worth writing.
So the habit is: name whose store, say which way it went, and name the other store that moved the opposite way. Three parts, every time.
Every store in this course is one of five kinds, and each belongs to something you can point at.
| Store | Fuller when | Example |
|---|---|---|
| moving | it goes faster | a rolling ball |
| raised | it is higher up | a book on a high shelf |
| stretched | it is stretched or squashed more | a wound-up spring |
| chemical | there is more food, fuel or charge | a full battery |
| thermal | it is warmer | a cup of hot cocoa |
Notice that each row says when the store is fuller. That is what lets you tell which way a transfer went: if the ball is slower afterward, its moving store went down. You never have to guess.
A transfer does not just happen; it travels by one of four paths.
Naming the path finishes the description. The battery's chemical store goes down and the bulb's thermal store goes up, by an electric current says what changed, which way, and how.
Because the down and the up are always the same amount, you can count them. Suppose a brick on a wall has $80$ units in its raised store. As it falls, the raised store empties and the moving store fills:
| Moment | Raised | Moving | Total |
|---|---|---|---|
| on the wall | 80 | 0 | 80 |
| halfway down | 40 | 40 | 80 |
| just before landing | 0 | 80 | 80 |
The last column never changes. That is the most important fact about energy, and scientists have tested it in millions of experiments. You will study it properly in two lessons' time. For now, use it as a check: if your two stores do not add up to the starting total, one of them is wrong.
Some transfers swing back and forth. A child on a swing is highest at each end and fastest at the bottom. At the ends, her raised store is full and her moving store is empty. At the bottom it is the other way round. Every swing, the energy moves from raised to moving and back again.
A bouncing ball does the same thing with three stores. Falling, its raised store goes down and its moving store goes up. Squashing against the floor, its moving store goes down and its stretched store goes up. Then the squash springs back, the stretched store goes down, and the moving store fills again as the ball leaps up.
Each step is still one store down and another up. A long story is just several transfers in a row, which is the subject of the next lesson.
Before you hand in a description of a transfer, run four checks on it.
The second check is the one that catches the most mistakes. Asked about a car braking, many people say its moving store and its thermal store both go down. But the brakes get hot. The moving store goes down and the thermal store of the brakes goes up. If you ever find two stores moving the same way, look again: one of them must be going the other way.
A roller coaster car is pulled up the first hill by a chain. At the top it stops for a moment, high above the ground. Suppose it has $500$ units in its raised store and none in its moving store.
As it rolls down the first drop, the raised store goes down and the moving store goes up, by the force of gravity. At the bottom of the drop, nearly all $500$ units are in the moving store, and the car is going fastest. Climbing the next, smaller hill, the transfer runs backward: the moving store goes down and the raised store goes up again.
Engineers design every hill to be lower than the first one. That is because the car can never have more than the $500$ units it started with, so it can never climb higher than the first hill without the chain. Some units also leave through rubbing, which is the next part of the story.
When a tablet is plugged in to charge, an electric current carries the transfer from the wall socket into the tablet's battery. The battery's chemical store goes up. Suppose it starts at $20$ units and ends at $100$ units: it gained $80$ units.
Later, you watch a video. The battery's chemical store goes down, and the screen sends out light and the speaker sends out sound. After an hour the battery might be at $60$ units, so $40$ units have left it, by an electric current, to the screen and speaker. The tablet also gets a little warm: some of those units went into its thermal store.
Nothing was used up. The units went from the battery to the light, the sound and the warmth. The next two lessons follow them the whole way.
Two wrong pictures compete for this topic.
The first is the list: kinetic, potential, chemical, light, sound, heat, learned as labels and stuck on pictures. The trouble is that a label never changes, and everything interesting here is a change. Asked what happens when a ball is thrown up, a list-learner writes kinetic energy and stops, which is true at the bottom, false at the top, and no use anywhere.
The second is the liquid: energy as an invisible juice poured from one container into another, which gets used up like gasoline. It is a better picture than the list, and it is still wrong in one important way — nothing is used up, ever. The total is the same at the end as at the start. Where it all goes is the next lesson but one.
What to say instead is always the same shape: this store went down, that store went up, by this path. Say it that way every time, even when a shorter label is tempting, and the physics stays honest. A label can hide a mistake; a transfer sentence shows it at once.
Picture the moment before.
$\text{high up, not moving}$
Raised store full, moving store empty.
Picture the moment after.
$\text{low down, moving fast}$
Just before it lands.
Name the store that went down.
$\text{the brick's raised store}$
It got lower.
Name the store that went up.
$\text{the brick's moving store}$
It got faster.
Name the path.
$\text{by a force: gravity pulls it down}$
How it got across.
Picture the moment before.
$\text{food eaten, child standing still}$
The food's chemical store is full.
Picture the moment after.
$\text{child running}$
The child is moving now.
Name the store that went down.
$\text{the chemical store in the food}$
Used by the child's body.
Name the store that went up.
$\text{the child's moving store}$
The child is faster.
Name the path.
$\text{by forces from the muscles}$
Her legs push against the ground: not magic, a push.
Notice a second store going up.
$\text{the child's thermal store}$
Running warms you up too; the next lessons follow that part.
Start at the top of the drop.
$\text{raised } 60, \ \text{moving } 0$
Held still, high up.
Follow it halfway down.
$\text{raised } 30, \ \text{moving } 30$
Half the raised store has moved across.
Follow it to the floor.
$\text{raised } 0, \ \text{moving } 60$
Fastest just before it touches.
Follow the squash.
$\text{moving } 0, \ \text{stretched } 60$
The ball flattens and stops for an instant.
Follow the spring back.
$\text{stretched } 0, \ \text{moving } 60$
The squash pushes it back up.
Check each total.
$60 + 0 = 30 + 30 = 0 + 60$
The total never changes.
Describe one step properly.
$\text{moving down, stretched up, by a force}$
One store down, another up, and a path.
Name the store that went down.
$\text{the spring's stretched store}$
Something was wound up tight.
Name the store that went up.
Name the path.
Think about the moment when a child eats breakfast and then runs. Which kind of store is the one going **down**?
Complete the worked solution: a skateboarder at the top of a ramp has $24$ units in her raised store and none in her moving store. A quarter of the way down, how many units are in each store?
Find what has left the raised store.
$24 \div 4 =$ a units
A quarter of the way down, a quarter has moved.
Find what is left in the raised store.
$24 - \text{that} =$ b units
The raised store went down.
Fill the moving store.
$0 + \text{what left} =$ c units
What left the raised store arrived in the moving store.
When a candle burns down, three things can be named. Match each to what it is.
| the chemical store in the wax | the room's thermal store | by heating and by light | |
|---|---|---|---|
| the store that goes down | |||
| the store that goes up | |||
| the way the transfer happens |
A class wrote four sentences about a slingshot firing a pellet. Mark the one that actually describes a transfer.
This task has no paper form; do it on a device.
A ball is thrown straight up and caught again. Put these four moments in the order they happen.
Number the steps in order (write the number in the box):
For each event, write the store that empties and the store that fills. One word each: moving, raised, stretched, chemical or thermal.
| the store that empties (one word) | the store that fills (one word) | |
|---|---|---|
| a ball rolls up a slope and slows down | ||
| a wind-up toy runs across the floor | ||
| an elevator carries a load up to the top floor |
A ball is held at the top of a slope with $100$ units in its raised store and none in its moving store. Let it fall, a step at a time, until every unit has arrived in the moving store. One button breaks the rules; do not use it.
This task has no paper form; do it on a device.
A brick sits on top of a wall with $68$ units in its raised store and none in its moving store. It falls. Fill in both stores when it is a quarter of the way down, halfway down, three quarters of the way down, and just before it lands.
| raised store (units) | moving store (units) | |
|---|---|---|
| a quarter of the way down | ||
| halfway down | ||
| three quarters of the way down | ||
| just before it lands |
A garden lamp with a solar panel stands out all day and glows all evening. During the **day**, which way does the transfer go?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
For each event, write the store that empties and the store that fills. One word each: moving, raised, stretched, chemical or thermal.
| the store that empties (one word) | the store that fills (one word) | |
|---|---|---|
| a brick is dropped from a wall | ||
| a catapult's stretched band fires a pellet | ||
| a ball rolls up a slope and slows down |
You can name the store that empties, the store that fills and the path between them. Tell someone why the arrow has kinetic energy says less than it seems to.
17. Your turn: a wind-up toy runs across the floor. Which store goes down, and which up?, step 2
$\text{the toy's moving store}$
Something is now moving.
17. Your turn: a wind-up toy runs across the floor. Which store goes down, and which up?, step 3
$\text{by a force: the spring turns the wheels}$
How it got across.