Gravity Model Wiki

The Gravity Model holds that the potential use of a service at a particular location is directly related to the number of people in a location and inversely related to the distance people must travel to reach the service.

Main Points Right Here
According to the Gravity Model, consumer behavior reflects two patterns:
1.The greater the number of people living in a particular place, the greater is the number of potential customers for a service. A city block or apartment building that contains 100 families will generate more customers than a house containing only one family.
2.The farther people are from a particular service, the less likely they are to use it. People who live 1 kilometer from a store are more likely to patronize it than people who live 10 kilometer away.

In other words, cities, countries or regions or other things that have higher numbers of people and are close together are more likely to interact

Small cities or countries with low populations that are further away are less likely to interact.

So China and the US interact because, even though they are further apart, they are both large

The US has perhaps the most interaction with Mexico because both countries are large and close together

Whereas Nepal probably does not interact with Paraguay as they are both small and far away. You do this same thing with big and small cities

Best Location in a Nonlinear Settlement
-Most settlements are more complex than a single main street. Geographers still apply the gravity model to find the best location, following these steps:
1.Identify a possible site for a new service.
2.Within the range of the service, identify where every potential user lives.
3.Measure the distance from the possible site of the new service to every potential user.
4.Divide each potential user by the distance to the potential site for the service.
5.Sum all of the results of potential users divided by distances.
6.Select a second possible location for the new service, and repeat steps 2, 3, 4, and 5.
7.Compare the results of step 5 for all possible sites. The site with the highest score has the highest potential number of users and is therefore the optimal location for the service.

While the gravity model was created to anticipate migration between cities (and we can expect that more people migrate between LA and NYC than between El Paso and Tucson), it can also be used to anticipate the traffic between two places, the number of telephone calls, the transportation of goods and mail, and other types of movement between places. The gravity model can also be used to compare the gravitational attraction between two continents, two countries, two states, two counties, or even two neighborhoods within the same city.
Opponents of the gravity model explain that it can not be confirmed scientifically, that it's only based on observation. They also state that the gravity model is an unfair method of predicting movement because its biased toward historic ties and toward the largest population centers. Thus, it can be used to perpetuate the status quo.

The Gravity Model, based off of Newton’s Law of Gravity, is used to measure the movement of ideas and people between two places. In this model distance between two areas and their population sizes are taken into consideration.

There is a general “formula” used when determining the “gravitational pull” a city has.

Population of area 1 x Population of area 2

Things such as employment, wages, settlement locations, traffic, trade, migration, and other ideas can be discovered using this model.

A downside and often criticism of this model is that there technically is no theoretical support, it is merely an observation.

Reilly’s Law of Retail Gravitation is a variation of this theory/model. Reilly used this model in order to establish the hinterland around a city. For example, the hinterland of two cities of the same size would have a trade area boundary exactly in the middle of each city. In other cases, the smaller the city, the closer the hinterland. The boundary between both cities is known as the breaking point. At this point, half of the populations shop in either city.
Faults related to this model, as with many others, are its assumptions. Reilly’s assumptions include a land void of any geographical variances, consumer preferences; political boundaries that effect an individuals “progress” towards a city.

Distance decay is directly relatable to this model and law.
Distance decay states that the further away two cities or ideas are, the less likely they are to interact and move.