The universe has a lot of forces, a lot of pushes and pulls. We're always pushing or pulling something, even if only the ground. But it turns out that in physics, there are really only four fundamental forces from which everything else is derived: the strong force, the weak force, the electromagnetic force, and the gravitational force.
The gravitational force is a force that attracts any two objects with mass. We call the gravitational force attractive because it always tries to pull masses together, it never pushes them apart. In fact, every object, including you, is pulling on every other object in the entire universe! This is called Newton's Universal Law of Gravitation. Admittedly, you don't have a very large mass and so, you're not pulling on those other objects much. And objects that are really far apart from each other don't pull on each other noticeably either. But the force is there and we can calculate it.
Sir Isaac Newton
Sir Isaac Newton
Universal Gravitation Equation
This equation describes the force between any two objects in the universe:
Universal Gravitation Equation
In the equation:
F is the force of gravity (measured in Newtons, N)
G is the gravitational constant of the universe and is always the same number
M is the mass of one object (measured in kilograms, kg)
m is the mass of the other object (measured in kilograms, kg)
r is the distance those objects are apart (measured in meters, m)
So if you know how massive two objects are and how far they are apart, you can figure out the force between them.
Inverse Square Law
Notice that the distance (r) on the bottom of the equation is squared. This makes it an inverse square law. Because of this, if you double the distance between two objects, you reduce the gravitational force between them to a quarter of what it was. Or if you triple the distance between them, you reduce the force to a ninth of what it was. Or if we go the other way, halving the distanc