Two lines and one crossing point, and almost everything else in economics hangs off them. Buyers want more when the price falls; sellers want to make more when it rises; and the price nobody chose ends up being the one where those two agree. The hard part is not the graph — it is telling the difference between a slide ALONG a curve and the whole curve MOVING, because a question you read backwards you will answer backwards. Five tabs — cards for the vocabulary, hints for the shifters, a quiz built on the along-or-shift question, and a workshop where you graph a real schedule, sort what shifts which curve, put a market in order and argue a price.
Buyers. The lower the price, the more they want — so this line runs downhill. Price up one side, quantity along the bottom.
Everything in this unit is one of two moves. If the GOOD'S OWN PRICE changed, you slide to a different point on the SAME line — that is a change in quantity demanded. If ANYTHING ELSE changed — income, tastes, the price of something related, expectations, the number of buyers or sellers — the whole line MOVES, and that is a change in demand or supply. Ask which one the question changed before you draw a single thing.
What people get wrong
⚠️People often think…
A rise in the price of coffee shifts the demand curve for coffee.
Its own price moves you ALONG the curve and never shifts it. A rise in the price of TEA would shift coffee's demand, because tea is something else. This one distinction is behind about half the questions in this unit, wearing a different hat each time.
Own price: along. Everything else: shift.
⚠️People often think…
A shortage pushes the price down, because that is what people need.
It pushes the price UP. More people want it than can be served, so buyers compete with one another and whoever will pay more gets it. A surplus does the opposite. Get this direction backwards and every prediction that follows is backwards too.
Shortage pushes up. Surplus pushes down.
⚠️People often think…
“Demand went up” and “quantity demanded went up” are two ways of saying the same thing.
They describe two different pictures. Demand is the whole curve; quantity demanded is one point on it. “The price fell, so demand rose” is a wrong sentence even when the student understands the economics perfectly — and on a written answer, the sentence is what gets marked.
Curve: demand. Point on it: quantity demanded.
⚠️People often think…
The equilibrium price is the fair price.
Equilibrium says where the market settles, not whether the outcome is good. A demand curve counts willingness AND ABILITY to pay, so somebody who desperately needs the thing but cannot pay the clearing price is simply not on the curve. The market has cleared; they are still outside it. Both sentences are true at once, and an honest answer says so.
Cleared is not the same as everyone served.
Worth knowing cold
The pairs worth knowing cold
Four pairs cover most of the mistakes in this unit. Learn each as two opposites, never as one blur.
Along / shiftThe good's OWN price slides you along its curve. Anything else moves the whole curve. Decide which one the question changed before you draw.
Demand / quantity demandedDemand is the whole curve. Quantity demanded is one point on it. The words are not interchangeable and the difference is marked.
Surplus / shortageAbove equilibrium: too much is made, the pile grows, the price falls. Below it: too little is made, a queue forms, the price rises.
Substitute / complementA substitute is the one you switch TO, and its price moves your curve the same direction. A complement is the one you buy WITH, and its price moves your curve the opposite way.
Read it, name it, move it, say it
1Watch one
A new factory halves the cost of aluminum. Work out what happens in the market for bicycles.
Ask first: did the price of a BICYCLE change? No. So nothing slides along a curve.
Aluminum is an input, so this is a supply shifter, not a demand shifter.
Cheaper to make means more will be made at every price: supply shifts RIGHT.
Supply right with demand unchanged: the price falls and the quantity traded rises. ✓
2Do one with me
Fill in the word each sentence is missing.
A price set BELOW equilibrium produces a
A rise in the price of a SUBSTITUTE shifts this good's demand which way?
A change in the good's own price changes the quantity demanded, not the
💬One sentence, then you move on
Why does a rising price tell a producer something, even though nobody sent a message?
3Try one
Demand shifts RIGHT and supply shifts LEFT at the same time. One of price and quantity is certain and the other is not. Which one is CERTAIN?
I want a hint first
Demand right on its own: price up, quantity up. Supply left on its own: price up, quantity down. Now put them side by side — which column agrees with itself?
💬Last one — then you're done here
A market clears, and some people still go without the thing. How can both be true?
Where this goes
Where this lives
Why concert tickets resell for triple, why gas prices move the week something happens on the other side of the world, why the thing everyone wants is the thing nobody can get at the listed price.
What this feeds
Next: what happens when someone decides the market's answer is the wrong one and holds the price somewhere else by law.
Name one thing you wanted recently that was sold out, and say what the price was telling you.
One card at a time — tap “Show me” to check yourself, then Next. Start at Foundation; when those feel easy, climb.
Helpful Hints
🧭 The unit in one line
Buyers want more as the price falls → sellers want to make more as it rises → the price settles where those two quantities match → and a price away from that point leaves either a pile nobody bought or a line nobody can be served.
Down for buyers, up for sellers, and they meet in the middle.
🔑 The one idea
The good's OWN price moves you ALONG its curve. Anything else that changes — income, tastes, the price of something related, expectations, the number of buyers — SHIFTS the whole curve. Half the questions in this unit are that one distinction wearing a different hat.
⚠️ Traps to avoid
A rise in coffee's price does NOT shift the demand for coffee. Its own price slides you along the curve. A rise in TEA's price shifts coffee's demand, because tea is something else.
“Demand” and “quantity demanded” are not the same words. Demand is the whole curve. Quantity demanded is one point on it. Using them interchangeably is how a correct answer turns into a wrong sentence.
A shortage pushes the price UP, not down. More people want it than can get it, so buyers bid against each other. A surplus pushes it down. Get the direction wrong and every prediction after it is wrong too.
Equilibrium is not the fair price. It is where the market settles. A market can clear at a rent nobody in town can pay, and it is still equilibrium.
A sale, a coupon or a discount is the good's own price. It moves you along the curve; it does not shift demand.
When BOTH curves shift, one of price or quantity becomes uncertain. Say which one you cannot determine instead of guessing — that is the answer the question wants.
📐 What shifts which curve
Change
Shifts
Which way
Incomes rise (normal good)
Demand
Right
A substitute gets cheaper
Demand
Left
A complement gets cheaper
Demand
Right
More buyers in the market
Demand
Right
Input costs rise
Supply
Left
Better technology
Supply
Right
A new tax on producers
Supply
Left
The good's own price
Neither
Moves along
Own price: along. Everything else: shift.
🎯 How you will be asked
“Does this move along the curve or shift it? Which curve, and which way?”
“At $6, 55 are demanded and 30 supplied. What is the size of the shortage?”
“Demand rises and supply is unchanged. What happens to price and quantity?”
“Both curves shift right. Which of price or quantity cannot be determined?”
“Is this pair substitutes or complements?”
✅ Before the test, can you…
State both laws in one sentence each, with “other things equal” in them?
Name four things that shift demand and four that shift supply?
Read a schedule and find the equilibrium price without a graph?
Say which way price moves when there is a shortage, and why?
Explain what a rising price tells a producer, in one sentence?
🧠 Worth knowing cold
Along / shift — the good's own price against everything else.
Demand / quantity demanded — the whole curve against one point on it.
Surplus / shortage — too much at too high a price against too little at too low a price.
Substitute / complement — the one you switch TO against the one you buy WITH.
Pick your level
Look back at anything you missed — the hint that appeared is exactly what to reread tonight.
How sure did you feel?
Workshop
Work like an economist: graph a real demand schedule and find the price that clears it, sort twelve changes by which curve they move, put one market's adjustment in order, then argue a price somebody will be angry about.
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