Pizza Demand and the Law of Demand

Demand Schedule and Curve

  • Adam’s pizza decision is used to study the relationship between price and quantity demanded.
  • Concept: demand schedule – pairs of price and quantity demanded for pizza (in slices).
  • We keep all other factors constant (ceteris paribus). This latin phrase (ceteris paribus) means all else equal; it’s used to focus on the effect of the pizza price only.
  • Axes on the graph:
    • Price of pizza on the y-axis (in dollars).
    • Quantity of pizza (slices) on the x-axis.
  • Sample data points (the numbers are illustrative):
    • At price $1 per slice, Adam wants 8 slices:
    • Point: $(p,q) = (1, 8)$.
    • At price $2 per slice, Adam wants 5 slices:
    • Point: $(p,q) = (2, 5)$.
    • At price $3 per slice, Adam wants 4 slices:
    • Point: $(p,q) = (3, 4)$.
    • At price $4 per slice, Adam wants 3.5 slices (not necessarily whole):
    • Point: $(p,q) = (4, 3.5)$.
  • Connecting these points yields Adam’s demand curve.
  • Important distinction: this curve shows willingness to pay for any given quantity, not the actual price paid.
  • What the curve tells us about marginal benefit:
    • For the fifth slice, Adam’s marginal benefit (the amount he’s willing to pay for that additional slice) is $2.0:
    • $MB_{5} = 2$ (in dollars).
    • The fact that he buys 5 slices at $2 and 4 slices at $3 indicates this marginal benefit relationship.
  • The demand curve is generally downward-sloping: higher price → lower quantity demanded; lower price → higher quantity demanded.
  • Why the slope? Diminishing marginal benefit:
    • The first slice is worth a lot to Adam; it’s highly satisfying.
    • The second slice is still good, but not as satisfying as the first.
    • The eighth (or later) slice is much less valuable because he’s closer to being full.
    • Hence, the willingness to pay for each additional slice declines with more slices consumed: “diminishing marginal benefit.”
  • Key takeaway about MB and demand:
    • The curve measures willingness to pay on the margin for each additional unit; higher for the first slice, lower for later slices.
    • The shape of the curve depends on the chosen numeric example; it need not be perfectly linear.
  • Substitution vs. income effects (when price changes):
    • Substitution effect: when pizza gets cheaper relative to other goods, you substitute towards pizza (more pizza purchased as it becomes relatively cheaper).
    • Income effect: when pizza becomes cheaper, your purchasing power increases (e.g., with a fixed budget you can buy more pizza overall).
    • Example of income effect: with $20 total, you can buy
    • $ rac{20}{4} = 5$ slices at $4 each, or
    • $ rac{20}{2} = 10$ slices at $2$ each.
    • These effects together explain why a price drop leads to more pizza bought.
  • Total effect and the law of demand:
    • Combined income and substitution effects drive an increase in quantity demanded when price falls, and a decrease when price rises.
    • The law of demand holds under normal conditions, with a few exceptions.
  • Individual demand perspective:
    • The question “Are you willing to pay for another slice?” depends on your current consumption; the answer for each additional slice defines your individual demand curve.
  • Summary concepts:
    • Demand schedule: price vs. quantity demanded.
    • Demand curve: the graphical representation of the schedule; downward-sloping.
    • Willingness to pay (marginal benefit): the maximum price you would pay for an additional unit.
    • Diminishing marginal benefit: MB declines as quantity increases.
    • Substitution effect: price change alters relative attractiveness of goods.
    • Income effect: price change alters purchasing power.
    • Total effect: combination of substitution and income effects.
    • Ceteris paribus: holding everything else constant to isolate the effect of price on quantity demanded.

Key terms and formulas

  • Marginal benefit for the nth slice: MB_n, typically decreasing in n.
  • Example from the transcript: MB_5 = 2 (the fifth slice is worth $2 to Adam at the margin).
  • Relationship between price and quantity (law of demand):
    • As price falls, quantity demanded increases: rac{dq}{dp} < 0 (holding everything else constant).
  • Budget/purchasing power example (income effect):
    • If budget is B and price is p, quantity affordable by income effect is approximately q = rac{B}{p} (illustrative for the example with $B = 20$).
  • Substitution effect intuition: a fall in price makes pizza cheaper relative to other goods, shifting consumption toward pizza.
  • Market relevance: these concepts extend beyond a single person (Adam) to the overall demand in a market, with the aggregate demand curve being the sum of individuals’ demands.