Gravity
Applies a strong downward force to particles.
It is simply a predefined 
Force node whose force and direction can be altered in the parameter editor. This tool is used to apply gravity to particles, which makes them appear as if they are falling naturally.
Using Gravity
Click the
Gravity button on the Drive Particles tab.Click
the 
particle system you want to be affected and press Enter to confirm your selection.Specify the node settings in the operation controls toolbar or parameter editor.
For specific parameter help see the Gravity node help.