Gravity Model

The Gravity Model is a formula that predicts the amount of interaction (migration, trade, communication) between two places, where bigger populations attract more interaction and greater distance reduces it. In AP Human Geography, it's one of the principles in EK PSO-6.C.1 for explaining how cities interact.

What is the Gravity Model?

The Gravity Model borrows Newton's idea of gravity and applies it to places. Two things determine how much two locations interact, their size and the distance between them. Bigger places pull harder. Farther apart means weaker pull. The basic formula multiplies the populations of two places and divides by the distance between them (often distance squared), so a pair of large cities close together should have far more migration, phone calls, trade, and commuters flowing between them than two small towns hundreds of miles apart.

In the CED, the Gravity Model lives in Topic 6.4 as one of the principles useful for explaining the distribution, size, and interaction of cities (EK PSO-6.C.1), alongside the rank-size rule, the primate city concept, and Christaller's central place theory. But it's really a workhorse model you can apply anywhere flows exist. Why do more migrants move from Mexico City to Los Angeles than to Boise? Gravity Model. Why does a big regional mall draw shoppers from farther away than a corner store? Gravity Model. It's distance decay with size added in, which is exactly why it shows up across so many units.

Why the Gravity Model matters in AP Human Geography

The Gravity Model is named explicitly in EK PSO-6.C.1 under learning objective 6.4.A, which asks you to use concepts like hierarchy, interdependence, relative size, and spacing to explain how cities are distributed and how they interact. That makes Topic 6.4 its home base. But its real exam value is that it quantifies two Unit 1 spatial concepts from Topic 1.4, distance decay and flows. It also explains migration patterns in Unit 2 (why pull factors from Topic 2.10 work more strongly on nearby, large destinations) and the spread of goods and ideas tied to cultural diffusion in Topic 3.6. If a question asks you to predict where people, products, or information will flow, the Gravity Model gives you a two-variable answer, size and distance.

How the Gravity Model connects across the course

Distance Decay (Unit 1)

Distance decay says interaction weakens as distance grows. The Gravity Model is distance decay turned into math, with population size added so you can compare specific pairs of places instead of just saying 'farther equals less.'

Central Place Theory (Unit 6)

Both appear in EK PSO-6.C.1 as tools for explaining urban systems. Christaller maps out where settlements and services should locate, while the Gravity Model predicts how strongly any two of those settlements will interact. Christaller draws the network; gravity measures the flows on it.

Push and Pull Factors in Migration (Unit 2)

Topic 2.10's pull factors explain why people want to move somewhere, and the Gravity Model explains why they usually pick the big city nearby over the bigger city far away. Distance acts like an intervening obstacle, draining a destination's pull the farther away it is.

Contemporary Causes of Cultural Diffusion (Unit 3)

Topic 3.6 covers how communication technology and time-space convergence accelerate cultural interaction. The internet effectively shrinks the distance term in the gravity equation, so size matters more and distance matters less, which is why global culture flows out of huge media hubs.

Is the Gravity Model on the AP Human Geography exam?

Expect the Gravity Model on multiple-choice questions tied to Topic 6.4, where it sits next to the rank-size rule, primate cities, and central place theory in EK PSO-6.C.1. A classic stem describes two pairs of cities with different sizes and distances and asks which pair has the most interaction, or asks you to identify which urban concept explains a flow pattern. Watch for distractors. Questions like 'which theory states a city's population is inversely proportional to its rank?' are testing the rank-size rule, not gravity, and questions about land values in the CBD are testing bid-rent. No released FRQ has used the term verbatim, but free-response questions on migration flows or city interdependence reward you for invoking it. Saying 'larger, closer places interact more because of the gravity model' is a clean way to earn an explanation point.

The Gravity Model vs Distance Decay

Distance decay is the simpler, one-variable idea that interaction drops off as distance increases. The Gravity Model includes distance decay but adds a second variable, population size. So distance decay alone can't explain why New York and Los Angeles, nearly 2,500 miles apart, interact more than two small towns 50 miles apart. The Gravity Model can, because the cities' massive sizes overpower the distance penalty. If a question only mentions distance, think distance decay; if it mentions size or population too, think gravity.

Key things to remember about the Gravity Model

• The Gravity Model predicts interaction between two places by multiplying their populations and dividing by the distance between them.

• Bigger places generate more interaction and greater distance reduces it, so two large, nearby cities will exchange the most people, goods, and information.

• It is named in EK PSO-6.C.1 (Topic 6.4) alongside the rank-size rule, the primate city, and central place theory as a principle for explaining city interaction.

• The Gravity Model is distance decay plus size, which is the key difference between the two concepts on multiple-choice questions.

• It applies beyond cities to migration flows (Unit 2), trade, and cultural diffusion (Unit 3), making it a strong cross-unit explanation on FRQs.

• Don't confuse it with the rank-size rule, which is about predicting a city's population from its rank, not about interaction between places.

Frequently asked questions about the Gravity Model

What is the Gravity Model in AP Human Geography?

It's a formula predicting the interaction between two places based on their population sizes and the distance between them. Larger populations increase interaction while greater distance decreases it, and the CED lists it in EK PSO-6.C.1 as a principle for explaining how cities interact.

What is the Gravity Model formula?

Interaction = (Population of Place 1 × Population of Place 2) ÷ Distance between them, with some versions squaring the distance. You won't need to calculate exact numbers on the exam, but you should be able to compare pairs of places using the logic.

Is the Gravity Model the same as distance decay?

No. Distance decay only says interaction weakens with distance, while the Gravity Model adds population size as a second factor. That's why two huge cities far apart can still interact more than two tiny towns that are close together.

How is the Gravity Model different from the rank-size rule?

The rank-size rule predicts a city's population from its rank in the urban hierarchy (the nth-largest city is 1/n the size of the largest). The Gravity Model predicts interaction between two places. Both appear together in EK PSO-6.C.1, so MCQs often use one as a distractor for the other.

Does the Gravity Model only apply to cities?

No. It home-bases in Topic 6.4 on city systems, but the same size-and-distance logic predicts migration flows in Unit 2, trade between regions, and the spread of cultural traits in Unit 3. Any flow of people, goods, or information is fair game.