How to use this page

This recap covers the first 37 minutes of the September 10 class, which picked up the elasticity topic at the second law of demand, where the September 8 class stopped, and finished it. (The September 3 opening and the September 8 class have their own recap and transcript pages, labeled Part 1 and Part 2.) The rest of the September 10 class opened the next topic, Gains from Exchange, and is on that lecture’s own pages. Read the three-step diagnostic first. Then use the lecture sections to reconnect each idea to an example from class. Finish with the application checks. The separate complete transcript is available when you want the full explanation, including the price-index question and the Civil War blockade story.

What you should be able to do

1. State the second law of demand, explain why the operative demand curve flattens as a price change persists, and name the margins (trips, carpools, transit, vehicles, commutes, homes) on which people adjust in the short run and the long run.
2. Convert two dollar prices into a relative price, say which good got cheaper in real terms when both dollar prices rise, and explain why a proportional change in all prices and incomes changes nothing.
3. Show, with numbers, how a fixed per-unit charge (shipping, a blockade, a babysitter) lowers the relative price of the higher-quality option, and use that to explain where the best produce is eaten.

Opening · A new practice tool

The table builder, and a word on studying with AI

Class opened with the graduate-program announcement (the instructor now runs the department’s master’s program; come talk to him about it) and a new practice tool built overnight at a student’s emailed request: “Practice: where every number in the elasticity table comes from,” in the Lecture 3 module. It opens on one solved step, walks you through any other step on request, and lets you enter the numbers yourself once you are ready. Two suggestions for studying: use AI tools for every class, not just this one, and if you have the Claude or ChatGPT browser plugin, open a lecture transcript page and ask it to make flashcards over the concepts. Send the instructor anything you build; a student’s biology-exam game got a mention.

Movement 03 · Time, continued

The longer a price change has held, the bigger the response

Picking up where Tuesday stopped: gas goes up 10%. Why might your driving next week look just like this week’s, and why might that change over six months or a year? Because adjusting takes time and depends on how long the change is expected to last. Nobody sells a car to get through one expensive week. In the short run you go to the closer grocery store; in the long run you might change your car, move closer to work, take a job that lets you work from home three days a week, or ride transit. So the long-run demand for gasoline is more elastic than the short-run demand, because more options are open to you. The first TopHat item asked which pattern a 10% fuel-price rise produces over time; most of the room picked the wrong one at first (“no response occurs unless every driver changes behavior immediately”), and the right one is small trip reductions first, then carpooling and transit, then eventually a different vehicle or commute. A lesson attached to one distractor: habits are not fixed by nature. They depend on costs and benefits, and how much people drive depends on the price of driving.

On the slide the price of gasoline falls from P₁ to P₂ and people buy more; over time the operative demand curve flattens, because you do not buy a Suburban the day gas gets cheap, but you might if the low price is expected to last, and you might move farther out and buy the bigger house. Run it in reverse for a price rise and the short-run curve is steep while the long-run curve is flat. The second TopHat item asked why purchases keep rising at the unchanged lower price after several months: the operative demand curve becomes flatter, producing a larger quantity response. A quick check attached to it: a change in the price of gasoline is a slide along the curve, not a shift of it.

Movement 04 · The real price is the relative price

Candy costs more dollars. Did it get more expensive?

What do economists mean by a price? Not the dollar price. Suppose you spend all your income on apples and oranges, and the price of each doubles along with your income. Nothing changes: you can buy exactly what you bought before, and the price of apples in oranges is what it was. In micro the price that matters is the real price, meaning the relative price: what you give up of one good to get the other. When you hand over $10 for gas, what you really give up is whatever else the $10 would have bought.

The worked case: candy goes from $4 to $5 a pound, ice cream from $2 to $3 a quart. Cancel the dollars. Before, a pound of candy cost 2 quarts of ice cream; now it costs 1.67 quarts. Candy’s dollar price rose and its real price fell. Relative prices are ratios, so flip it: a quart of ice cream used to cost half a pound of candy and now costs 0.6 pounds. If candy got cheaper in ice cream, ice cream got dearer in candy, by definition. A student asked how this works in an economy with more than two goods: the Bureau of Labor Statistics’ Consumer Price Index and the Bureau of Economic Analysis’ PCE price index are averages of everything, and a real gas price is the gas price against that average. That is macro, and this course will not go there. Two closing points: inflation can change relative prices in the short run because some prices (grocery shelves) adjust faster than others (wages), so a real wage can fall until wages catch up; and for the rest of the semester, if you think the answer to a question is “inflation,” you are wrong.

Why is so much of Maine’s best lobster eaten in Chicago?

If you want the best Maine lobster, should you go to Maine? Most people say yes; economics says no, and the reason is a relative price. Regions that produce the best grapes or the best lobster tend to ship the good ones out and keep the average ones. Choice grapes are $1.00 a pound in California and standard grapes $0.50, so a pound of choice grapes costs two pounds of standard. Shipping either kind to Chicago costs the same $0.50 a pound. At the destination the prices are $1.50 and $1.00, and a pound of choice grapes now costs only 1.5 pounds of standard. The fixed charge lowered the relative price of the better good, so the destination buys the good grapes, and the same logic ships out the good lobster and the good leather. Do not buy an Italian leather jacket in Italy: the better jacket is relatively cheaper here. The TopHat item asked why shipping costs lead a region to export its highest-quality produce, and the room had it: a fixed shipping charge reduces the relative price difference between high and standard quality in the destination market. The opposite claim, that it widens the difference, is exactly backwards.

The same tool solves a history puzzle. Lincoln’s Anaconda Plan blockaded the Confederacy from Virginia to New Orleans, and running the blockade risked losing the whole cargo whether it was socks or chocolate and lace doilies. A fixed charge on every shipment lowers the relative price of the luxury goods, and that is what came in, so in 1864 the Confederate Congress taxed any ship whose cargo was not at least three-quarters necessities. And a smaller puzzle: couples with children spend their night out on a fancy dinner or the opera more often than childless couples do, because the babysitter costs the same either way and lowers the relative price of the expensive evening. A student asked whether freshness, not price, keeps the best lobster in Maine. The instructor’s answer: maybe a hundred years ago, but today Japanese fish reaches Michelin-star sushi counters within the day and Italian truffles are sold in New York the morning they land, so a lobster caught today can be in Chicago for dinner tonight.

The topic is complete

This was the last of three sessions on elasticity and the applications of demand. The deck’s closing summary slide was shown and passed over (“this is just a summary of the current one”), so the four tools it names are worth restating here. Elasticity: two percentages on midpoint bases, reported for a price interval, never the slope. Revenue: where demand is elastic quantity wins, where it is inelastic price wins, and the frost can be read backwards. Time: the response keeps growing after the price moves, so put a clock on each margin. Relative price: always ask “relative to what?”, and remember that a fixed charge favors the dear grade. At 36:40 the class moved on to Gains from Exchange, which has its own recap and transcript in the Lecture 4 module. Reminder from the top of class: the practice tools in the Lecture 2 and Lecture 3 modules regenerate endlessly and do not affect your grade; if another tool would help, email the instructor.

Examples are memory cues; the right column is the principle each example should help you recover.

Example from class	Economic lesson
Selling the car to get through one expensive week	Adjustment depends on how long the price change is expected to last; the expensive adjustments wait for a change expected to persist.
The closer grocery store now; the new car, job, or house later	The second law of demand: cheap margins move first, expensive margins later, so the operative demand curve flattens with time.
Buying the Suburban once cheap gas is expected to last	The law runs both ways: a price cut expected to hold also draws a bigger response over time.
“No response unless every driver changes at once”	Each driver responds to the price on his own margins; habits are not fixed by nature, they depend on costs and benefits.
Apples and oranges double, and so does your income	A proportional change in every price and income changes no relative price and no choice; only relative prices govern behavior.
Candy $4 to $5, ice cream $2 to $3: 2 quarts to 1.67 quarts	The dollar tag can rise while the real price falls; cancel the dollars and read what you give up in the other good.
A quart of ice cream: half a pound of candy, then 0.6 pounds	Relative prices are ratios; if one good got cheaper in the other, the other got dearer in it, by definition.
Grocery prices move before wages do	Inflation can change relative prices in the short run because prices adjust at different speeds; that is a macro story, and it is never the answer in this course.
Choice grapes $1.00, standard $0.50, plus $0.50 shipping each	A fixed per-unit charge lowers the relative price of the higher-quality good (2 to 1.5), so the best produce is shipped out: the Alchian–Allen effect.
Maine lobster in Chicago; the Italian jacket bought at home	Where the best quality is eaten or worn is a relative-price question, not a freshness or patriotism question.
The Anaconda Plan and the 1864 necessities law	A blockade is a fixed charge on every cargo; it lowers the relative price of luxuries, which is why chocolate and lace came in and socks did not.
The babysitter and the opera	Any fixed cost attached to both options lowers the relative price of the expensive one; the same effect in a different setting.

Question 1 — Gas rises 10% and is expected to stay there. Rank these responses from earliest to latest: buy a more efficient car, skip a discretionary trip, move closer to work, start carpooling.

Skip the trip, carpool, buy the car, move. The cheap adjustments come first and the expensive ones wait until the price change has been in effect long enough to justify them. That ordering is the second law of demand in action: the operative demand curve flattens as the horizon lengthens.

Question 2 — A classmate says the second law of demand is only about price increases. Is that right?

No. The law says the response to a price change, in either direction, grows the longer the change is expected to hold. The slide in class ran it for a price cut: nobody buys a Suburban the week gas gets cheap, but if cheap gas is expected to last, the bigger vehicle and the house farther from work eventually make sense.

Question 3 — After gasoline has been cheaper for several months, purchases keep rising even though the price has not moved again. Is that a shift of the demand curve?

No. It is the operative demand curve becoming flatter over time, producing a larger quantity response to the same price change. The price of gasoline changed, and a change in the good’s own price is always a slide along the curve; what changed with time is which curve is operative.

Question 4 — Coffee goes from $3 to $4 a cup and bagels from $1.50 to $2.50. Did coffee get more expensive?

No. Cancel the dollars. A cup of coffee used to cost 2 bagels (3 ÷ 1.50) and now costs 1.6 bagels (4 ÷ 2.50). Coffee’s dollar price rose and its real price, in bagels, fell. The same exercise as candy and ice cream.

Question 5 — In that same coffee-and-bagel case, what happened to the price of a bagel measured in coffee?

It rose, from 0.5 cups to 0.625 cups. Relative prices are ratios, so if coffee got cheaper in bagels, bagels got dearer in coffee. There is no way for both goods to get relatively cheaper at once.

Question 6 — Overnight, every price in the economy doubles and so does every wage. What happens to how much gasoline people buy?

Nothing. Every relative price is unchanged, and every income buys exactly what it bought before, so no choice changes. The price on a demand curve is the relative price. This is also why “inflation” is never the answer to a question in this course.

Question 7 — A $6 steak and a $3 burger each get the same flat $3 shipping charge added. Which becomes relatively cheaper at the destination?

The steak. At home a steak costs 2 burgers. At the destination it is $9 against $6, so a steak costs 1.5 burgers. The fixed charge narrows the relative-price gap, which is why the destination eats more steak. The same arithmetic as the grapes.

Question 8 — Chicago restaurants serve better Maine lobster than Maine restaurants do. A classmate says this proves Chicagoans just like lobster more. What is the economist’s explanation?

Hold tastes equal and the result still follows. Shipping costs the same per lobster whatever its quality, so in Chicago the best lobster is relatively cheaper than it is in Maine, where the price gap is at its widest. Buyers at the destination take the good ones and the average ones stay home. The Alchian–Allen effect; tastes are not needed.

Question 9 — Why would a wartime government facing a blockade pass a law taxing ships whose cargo is not mostly necessities?

Because the blockade itself is a fixed charge on every cargo, and a fixed charge lowers the relative price of luxuries. Blockade runners therefore brought in chocolate and lace rather than socks and leather, and the Confederate Congress in 1864 tried to tax the pattern away. Same principle as the grapes, in a different century.

Question 10 — A couple pays a babysitter $60 whether they go to a $40 movie night or a $160 dinner and opera. In what sense does the babysitter make the opera cheaper?

In the relative sense. Without the sitter, the expensive evening costs 4 movie nights (160 ÷ 40). With the sitter added to both, it costs 2.2 movie nights (220 ÷ 100). The dollar cost of the opera did not fall; its price relative to the alternative did, so couples with children choose it more often.

Bottom line

Elasticity is not a fixed property of a good. It grows with the time a price change is expected to hold, because the cheap adjustments come first and the expensive ones later. And the price it responds to is the relative price, what you give up in other goods, so a dollar tag can rise while the real price falls. Put a fixed charge on two goods and the better one gets relatively cheaper, which is why the best lobster leaves Maine, the best leather leaves Italy, and blockaded ports fill with luxuries. That completes the elasticity topic. The rest of the September 10 class opened Gains from Exchange.