Final answer:
The interaction constant in the gravity model is represented by the Social constant, which serves to balance the equation outputs similarly to the universal gravitational constant in physics, G. The gravity model is a geography concept that compares the interaction between two places to gravitational force. Hence, option D is correct.
Step-by-step explanation:
In the gravity model, the interaction or social 'constant' is represented by D) Social constant and is used as a mathematical way to get the interaction numbers to work out properly as the output of the equation. The gravity model is a theoretical model in the field of geography used to estimate the amount of interaction between two locations, often using the metaphor of gravity to describe how the strength of this connection diminishes with distance.
The gravity model typically involves a formula similar to that of Newton's Law of Universal Gravitation, where the gravitational force between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
The social constant in the context of the gravity model is analogous to G, the universal gravitational constant in physics, which has been experimentally determined to be a stable value throughout the universe, specifically G = 6.67 × 10^{-11} N · m^2/kg^2.
However, in the gravity model used for human interaction and economic studies, the 'social constant' is an abstract concept representing the strength of connection that is not tied to any physical properties.