What is the standard simple form of the gravity model equation used in geography?

What is the standard simple form of the gravity model equation used in geography?

• A. Interaction ∝ (P1 × P2) / (Distance)^2
• B. Interaction ∝ Distance^2 / (P1 × P2)
• C. Interaction ∝ (P1 + P2) × Distance
• D. Interaction ∝ Distance / (P1 × P2)

Answer: (A)

This question taps into how the gravity model maps how places interact: interactions rise with the size of each place and fall as distance grows. The standard simple form expresses this as I ∝ (P1 × P2) / (D^2). The product of the two populations reflects that interaction depends on both places acting together—larger places generate more potential flow. The distance in the denominator implements distance decay: as objects are farther apart, interaction drops, and squaring the distance makes that drop even stronger. A quick check: doubling one population increases predicted interaction, and doubling both increases it by a factor of four; increasing distance reduces interaction by a factor tied to the square of the distance. The other forms either make distance increase interaction or use an additive or inverted relationship that doesn’t capture the multiplicative effect of both places’ sizes or the inverse-square distance decay. So the standard simple form is the product in the numerator and the squared distance in the denominator.

This question taps into how the gravity model maps how places interact: interactions rise with the size of each place and fall as distance grows. The standard simple form expresses this as I ∝ (P1 × P2) / (D^2). The product of the two populations reflects that interaction depends on both places acting together—larger places generate more potential flow. The distance in the denominator implements distance decay: as objects are farther apart, interaction drops, and squaring the distance makes that drop even stronger. A quick check: doubling one population increases predicted interaction, and doubling both increases it by a factor of four; increasing distance reduces interaction by a factor tied to the square of the distance. The other forms either make distance increase interaction or use an additive or inverted relationship that doesn’t capture the multiplicative effect of both places’ sizes or the inverse-square distance decay. So the standard simple form is the product in the numerator and the squared distance in the denominator.