There are four fundamental forces: gravity, the electromagnetic force, the strong force and the weak force. Of these four, gravity is the weakest. You, however, live on a massive planet, so gravity is a very important force in your life and the one with which you are most familiar from everyday experience. The gravitational force an object exerts on other objects is proportional to its mass.

## Mutual Attraction

In classical physics, gravity is an attractive force that operates between all objects that have mass. Your table and your chair exert a gravitational pull on you just as your gravity exerts a pull on your table. When you stand next to a colleague, your gravity pulls on your colleague just as your colleague's gravity pulls on you. Any two objects that have mass will exert gravitational pull on each other, because any object with mass creates its own gravitational field.

## Strength

The gravitational attraction between most ordinary objects is very small. It becomes stronger as the masses of the two objects become greater, but the mass of an object has to become very great before it will create a strong gravitational field. The reason why Earth's gravity is so strong is because Earth is so massive. Other objects like the sun and other stars are far more massive still, so gravity is one of the most important forces for determining the structure of galaxies and stellar systems.

## Equation

Using Isaac Newton's Universal Law of Gravitation, you can calculate the strength of the force of gravity between two objects with a basic equation: force = G m1 m2 / r^2, where r is the distance between the two objects, m1 and m2 are their masses and G is a constant 6.67 x 10^-11 -- a very small number. As you can see, the two terms m1 and m2 are in the numerator of this equation, so the more you increase either of them, the more the force will increase.

## Earth

Assume that both you and a colleague weigh about 64 kg, a little more than 140 lb., and that you are standing 0.5 m apart. According to Newton's law, the force of the gravitational attraction between you will be about 1.09 x 10^-6 newtons or 2.46 x 10^-7 lb. -- so miniscule it might as well not even be there. Earth, however, has a mass of 5.974 x 10^24 kg, so the gravitational force that you and Earth exert on each other is 627 newtons, or about 140 lb.