The Physics controls how particles move once emitted. Here are properties such as gravity and turbulence and controls for making particles bounce on other layers in the composition.
Physics Model
There are two physics models available:
Air - this model is useful for making particles that move naturally through air. There are settings for air resistance, turbulence etc. Read more about Air.
Bounce - with this model particles can bounce on layers in the composition. Read more about Bounce.
Gravity
This control makes particles fall downwards in a natural way, picking up speed as it goes, just like any object in real world would.
Physics Time Factor
This parameter enables full control over time. It can be used to speed up or slow down the movements, or freeze completely, or play backwards. It is keyframable. Read more about Physics Time Factor.