State whether each of the following events will result in a movement along the demand curve for McDonald's Quarter Pounder hamburgers or whether it will cause the curve to shift. If the demand curve shifts, indicate whether it will shift to the left or to the right and draw a graph to illustrate the shift. a. The price of Burger King's Whopper hamburger declines. b. McDonald's distributes coupons for \(\$ 1.00\) off the purchase of a Quarter Pounder. c. Because of a shortage of potatoes, the price of French fries increases. d. McDonald's switches to using fresh, never-frozen beef patties in its Quarter Pounders. e. The U.S. economy enters a period of rapid growth in incomes.

A fundamental concept in demand curve analysis is the impact of substitute goods. When two products serve a similar purpose or satisfy the same need, they are considered substitutes. An easy way to understand this is with everyday items like soft drinks; a Coke and a Pepsi might both quench your thirst. If the price of Pepsi falls, people might buy more Pepsi instead of Coke.

In the context of our exercise, Burger King's Whopper is a substitute for McDonald's Quarter Pounder. When the price of the Whopper decreases, customers may opt for it over the Quarter Pounder due to its lower cost, ultimately reducing the demand for the latter. This economic behavior is visualized as a leftward shift in the demand curve for Quarter Pounders. It's not just a point moving along the curve—this is an entirely new curve, reflecting a change in demand at every price level.

Complementary goods, in contrast, are products that are often used together, where the use of one increases the use of another. Classic examples include peanut butter and jelly or printers and ink cartridges. When these items are used in conjunction, the demand for one can affect the demand for the other.

In our textbook problem, French fries are a complement to hamburgers. An increase in the price of fries leads to a decrease in the demand for fries and, by association, hamburgers. This phenomenon is seen as a shift to the left in the demand curve for Quarter Pounders. Students should understand that complementary goods have a direct relationship; if the cost or consumption of one rises, it typically affects the other in the same direction.

Another key factor that influences the demand curve is the income effect. Simply put, as people's income changes, so does their ability to purchase goods and services. Higher incomes generally lead to increased demand for goods because individuals have more disposable income to spend. Similarly, if incomes fall, consumers might cut back on purchases, particularly on non-essential items.

In the exercise, a period of rapid income growth in the U.S economy illustrates a positive income effect; as consumers' incomes rise, they are more likely to purchase Quarter Pounders, shifting the demand curve to the right. It's essential for students to connect the dots between income levels and consumer spending behavior, as it's one of the driving forces behind demand for products.

Lastly, consumer preference has a significant influence on the demand curve. Preferences can change due to trends, branding, perceived quality, and other factors. When preference for a product increases, so does the quantity demanded, even if the price remains constant. Conversely, if the product falls out of favor, the demand decreases.

In our scenario, McDonald's shift to using fresh, never-frozen beef patties could improve public perception of the quality of their hamburgers, which in turn can increase consumer preference for the Quarter Pounder. This change is hypothesized to cause a rightward shift in the demand curve, illustrating an increase in demand. It's crucial to encourage students to think about how quality improvements and marketing can affect public perception and demand.