The strength of the gravitational attraction between two objects depends on two factors: how big the objects are (how much mass they have) and how far apart they are. According to Isaac Newton’s law of universal gravitation, gravity’s strength is directly proportional to the product of two objects’ masses and inversely proportional to the square of the distance between them. To make this easier to understand lets break it down a little.
First, the force of gravity is directly proportional to the product of the masses. (Remember that a product is one number multiplied by another.) This means that the attraction between two objects gets stronger the larger the product of their masses is and weaker the smaller it is. Put even more simply, the bigger something is, the harder it pulls on other objects.
Second, the force of gravity is inversely proportional to the square of the distance between two objects. For those of you who haven’t taken algebra, the square of a number is that number multiplied by itself. The square of one is one, the square of two is four, the square of three is nine and so on. This means that if you double the distance between two objects their attraction will be only 1/4 its original strength and if you triple the distance, their attraction will be only 1/9 its original strength. This is called an inverse square law.
It is important to remember, however, that when we refer to the distance between two objects we are actually talking about the distance between their centers, not their surfaces. If that were true, gravity would pull 1/4 as hard on you two meters above that ground as it does one meter above the ground!
To summarize, the strength of the force of gravity between two objects gets larger the larger the objects are but smaller the farther apart they are.
For more on Sir Isaac Newton and the history of his law of universal gravitation see History Page 6.
