The gravity of your planet will affect the development of your life forms. Gravity will hold onto your atmosphere and will affect atmospheric pressure. It will determine how hard raindrops hit the ground as they fall, so it will affect rates of erosion.  it will determine how much force it will take to lift water up into your trees so that leaves will live. It will determine how heavy blood is, and how hard hearts will have to contract to make the blood circulate.
 
Gravity is the force that causes objects to attract one another. Scientists are studying it, and they can describe the way that it works. However, no one knows what it is or why it works as it does. We do know that an object with a lot of mass (like the sun!) has more attractive power than a small object. This power can be accurately calculated.

 
     Look at the picture of the circles above. You can see that the little gray circle in the center has a radius of 1. The yellow circle has a radius of 2 -- but look at how much yellow now surrounds the gray! The outside circle has a radius of 4. I have only doubled the radius, but look at how much more space this circle takes up!

This picture extends the circle into a three dimensional sphere.

There is a lot more planet when we increase the radius!

So if there is more matter, there will be more gravity, right?

Well, partly right. Density also has something to do with it.

Review the page on Weight, Mass and Density.

A planet made of metal will have a much higher density than one made of ice -- or styrofoam!!

This page will help you to figure out the gravity that there is on the surface of your world.

 
 The density of the earth is 5.5 grams per cubic centimeter.

Earth's density is unusually high for a planet in our solar system.

See Statistics on Planets and Moons for examples of other densities and radii.

Let's do some examples:
 

  Example One:

Your planet's density is 4.5

The density of earth is 5.5

Your planet's radius is 1.2 when compared to the earth's radius.

 Put in your own numbers now:

gravity of your planet = ( 4.5 )  multiplied by 1.2 =
                           5.5

.8  * 1.2 = 0.96 of earth's gravity

  The gravity on your planet is less than earth's gravity even though the radius is bigger.
  This is because the density of your planet is less than the density of the earth.