We may take it for granted now, but more than 300 years ago Sir Isaac Newton proposed a revolutionary idea: that any two objects, no matter their mass, exert gravitational force toward one another. This law is represented by an equation that many high schoolers encounter in physics class. It goes as follows:

**F = G Ã— [(m**_{1}**m**_{2}**)/r**^{2}**]**

**F** is the gravitational force between the two objects, measured in Newtons. **M**_{1} and **m**_{2} are the masses of the two objects, while **r** is the distance between them. **G** is the gravitational constant, a number currently calculated to be 6.672 Ã— 10^{-11} N m^{2} kg^{-2} [source: Weisstein].

The benefit of the universal law of gravitation is that it allows us to calculate the gravitational pull between any two objects. This ability is especially useful when scientists are, say, planning to put a satellite in orbit or charting the course of the moon.