Questions: Spatial Interaction and the Gravity Model

Question 1

According to the gravity model, if the distance between two cities doubles while their populations remain constant, how does predicted interaction between them change?

• A. It halves
• B. It falls to one-quarter of the original value
• C. It doubles
• D. It remains the same because populations are unchanged

Answer: (B)

The gravity model predicts interaction ∝ (M₁ × M₂) / d². If d doubles, d² quadruples, so the interaction is divided by 4 — falling to one-quarter of the original. Distance is squared in the denominator, which is why increasing separation has a compounding dampening effect on spatial interaction.

Question 2

Modern telecommunications and the internet have effectively eliminated the effect of geographic distance on most economic and social interactions.

• T. True
• F. False

Answer: False

Despite predictions of the 'death of distance,' geographic proximity continues to strongly influence interaction in most economic, social, and cultural domains. Agglomeration in cities, face-to-face business clustering, regional trade intensity, and local social ties all show persistent distance effects. Telecommunications reduces some friction of distance but does not eliminate it.

Question 3

What are Ullman's three conditions necessary for significant spatial interaction to occur between two places, and why is each condition necessary?

Model answer: Complementarity: one place must supply what the other demands, otherwise there is no motivation to interact. Absence of an intervening opportunity: no closer alternative must be available to satisfy the same need, or it will redirect the interaction. Transferability: movement must be physically and economically feasible — if transport costs are prohibitive, interaction cannot occur regardless of complementarity.

The gravity model captures mass and distance but not the content of why places interact. Complementarity explains the motivation; intervening opportunity explains why two complementary places may still have low interaction (a nearer substitute intercepts it); transferability sets a hard practical limit. All three must hold for meaningful spatial interaction to develop.