What is the gravity model of spatial interaction, and what two factors does it use to estimate flows between places?

• A. A model that estimates interaction based on the size of two populations and the distance between them; larger populations increase interaction, greater distance reduces it.
• B. It uses the sizes of the places, but distance is not a factor.
• C. A model that estimates interaction based on the size of two populations and the distance between them; larger populations increase interaction, greater distance reduces it.
• D. It ignores population size and uses only geographic proximity.

Answer: (C)

The gravity model treats flows between places much like gravity: bigger places pull more interaction, and places that are farther apart pull less. The two main ingredients are the sizes of the places (often their populations or economic mass) and the distance between them. In simple terms, flow increases with the product of the two places’ sizes and decreases as distance grows, commonly written as F ~ (P1 * P2) / D^β, where β controls how strongly distance dampens interaction. This captures why two large cities close to each other tend to have more trade, travel, or communication than smaller or more distant ones. The statement that best fits this idea is that interaction is based on the sizes of the places and the distance between them, with larger populations increasing interaction and greater distance decreasing it.