Lesson 01: Scarcity, Opportunity Cost & the PPC

Warm-Up

You have 4 free hours tonight. You could work your shift at $15/hour, study for tomorrow's quiz, or go to your friend's game. You can't do all three — the hours are scarce. Whatever you choose, you give something up. Question to hold in your head: what is the true cost of going to the game? (Hint: it's not zero, even though the game is free.) Economics starts exactly here — unlimited wants colliding with limited resources — and the AP exam's Unit 1 questions test whether you can make that collision precise with numbers and graphs.

Core Concept

Scarcity: the founding problem

Scarcity means resources are limited while wants are unlimited. Because of scarcity, every choice has a cost, and every economy — from your household to the U.S. — must answer three questions: what to produce, how to produce it, and for whom.

The resources being rationed are the factors of production:

Trap alert: in economics, capital means physical capital (tools, machinery) or human capital (skills) — not money. Money is a financial asset, not a factor of production. The AP exam loves this distractor.

Opportunity cost

The opportunity cost of a choice is the value of the next-best alternative you give up — not the sum of all alternatives, just the best one you didn't take.

Opportunity cost = what you sacrifice / what you gain

If you go to the game instead of working the 4-hour shift, your opportunity cost is $60 of earnings (assuming work was your next-best option). "Free" events are never free to attend.

Economists assume decision-makers are rational: they compare marginal benefit (MB) — the extra benefit of one more unit — with marginal cost (MC) — the extra cost of one more unit — and do something only when MB ≥ MC. This marginal-thinking thread runs through the entire course, so get comfortable with it now.

The Production Possibilities Curve

The PPC (also called PPF, production possibilities frontier) is Unit 1's model. It shows all the maximum combinations of two goods an economy can produce with fixed resources and technology.

[GRAPH: Production possibilities curve. X-axis: "Wheat (tons)". Y-axis: "Cars". A bowed-out (concave to origin) curve from (0, 100) on the Cars axis to (50, 0) on the Wheat axis. Point A on the curve at (20, 90); point B on the curve at (40, 50); point C inside the curve at (20, 40) labeled "Inefficient"; point D outside the curve at (45, 90) labeled "Unattainable".]

Read the graph: - On the curve (A, B): efficient — all resources fully employed. - Inside the curve (C): attainable but inefficient — unemployment or misallocation. - Outside the curve (D): unattainable with current resources and technology.

The two PPC shapes

1. Straight-line PPC → constant opportunity cost. Resources are equally suited to both goods. Every extra unit of X costs the same amount of Y.
2. Bowed-out (concave) PPC → increasing opportunity cost. Resources are specialized. As you produce more wheat, you must pull workers and machines that were better suited to making cars, so each extra ton of wheat costs more cars than the last. This is the law of increasing opportunity cost, and it's why most real-world PPCs bow outward.

How to compute opportunity cost on a PPC: it's the slope between two points. Moving from A (20, 90) to B (40, 50): you gain 20 wheat, you give up 40 cars → each ton of wheat costs 40/20 = 2 cars.

Shifting the PPC

The whole curve moves when the economy's productive capacity changes:

Key distinctions the exam tests: - Moving from inside the curve to the curve = using existing capacity (reducing unemployment). Not growth. - The curve itself shifting outward = economic growth. - A technology improvement affecting only one good (say, cars) rotates the curve outward on the car axis only — the wheat intercept stays put. - Choosing a point with more capital goods today (vs. consumer goods) shifts the future PPC out farther — investing in capital fuels growth.

Economic systems (brief)

Who decides what/how/for whom? - Market economy: decentralized decisions by buyers and sellers via prices (the "invisible hand"). - Command economy: central government decides. - Mixed economy: markets plus government (every real economy, including the U.S.).

Worked Examples

Example 1 (easy): Opportunity cost of college

Maya pays $12,000/year tuition and could have earned $30,000/year working full-time. Books cost $1,000. She would pay $9,000 for rent and food whether or not she attends college. What is her annual opportunity cost of attending?

Strategy: Opportunity cost = explicit costs caused by the choice + forgone earnings. Exclude costs paid either way.

Solution: Tuition $12,000 + books $1,000 + forgone earnings $30,000 = $43,000. Rent and food are excluded — she pays them in both scenarios, so they're not sacrificed by choosing college.

Interpretation: The exam's favorite trick is including expenses that occur under every alternative. Only count what the choice itself costs you.

Example 2 (medium): Per-unit opportunity cost from a table

Country Z can produce the following combinations:

(i) What is the opportunity cost of the first 10 fish? (ii) The last 10? (iii) Is opportunity cost constant or increasing?

Solution: - A→B: gain 10 fish, lose 60 − 45 = 15 coconuts → 1.5 coconuts per fish - C→D: gain 10 fish, lose 25 − 0 = 25 coconuts → 2.5 coconuts per fish - Cost per fish rises (1.5 → 2.0 → 2.5) → increasing opportunity cost, so the PPC is bowed out.

Interpretation: Always compute cost per unit gained. If the ratio changes as you move along the table, the curve is concave.

Example 3 (AP-style): Shifts vs. movements

The economy of Alta produces capital goods and consumer goods and is currently operating on its PPC. (i) Alta's unemployment rate rises sharply. Show the effect. (ii) Instead, a new technology improves the production of capital goods only. Show the effect. (iii) If Alta then chooses a point with more capital goods, what happens to future growth?

Solution: - (i) The curve does not move. Production moves from a point on the curve to a point inside it. - (ii) The curve rotates outward along the capital-goods axis; the consumer-goods intercept is unchanged. - (iii) More capital today → more productive capacity tomorrow → the future PPC shifts outward farther than it otherwise would.

Interpretation: "Unemployment" is never a shift of the PPC — that's the single most common Unit 1 FRQ error.

Common Mistakes

1. Calling money "capital." Wrong: money finances purchases of capital. Right: capital = tools, machines, factories, human skills.
2. Adding up all forgone alternatives. Opportunity cost is the next-best single alternative, not everything you could have done.
3. Shifting the PPC for unemployment. Unemployment moves the economy inside the curve; the curve moves only when resources or technology change.
4. Ignoring which axis a technology change affects. A one-good technology improvement rotates the curve on that good's axis only.
5. Counting unavoidable expenses in opportunity cost. If you pay it under every alternative (like Maya's rent), it's not part of the cost of the choice.

Practice Problems

11. (FRQ-style) The nation of Corvia produces only trucks and grain, with specialized resources. (a) Draw a correctly labeled PPC for Corvia consistent with increasing opportunity costs. (b) On your graph, label a point E that is efficient, a point U that reflects unemployment, and a point N that is currently not attainable. (c) Corvia's engineers develop a fertilizer that doubles grain yields but does not affect truck production. Show the effect on your graph. (d) Explain how point N could become attainable in the future.