The Gravity Model is a model used to estimate the amount of interaction between two cities. It is based on Newton's universal law of gravitation, which measured the attraction of two objects based on their mass and distance. The gravity model for cities works in the same way; the amount of interaction between two cities is proportional to the size of the cities and inversely proportional to their distance squared. This explains why cities that are far apart, such as New York and Los Angeles, still interact a lot despite being on either side of the United States, thousands of miles apart.