Creating a Game Menu and a Particles Effect: An Angry Birds-like Game, Part II - Learn GameSalad for iOS: Game Development for iPhone, iPad, and HTML5 - page 201

Click the Home button and then click the Actors tab. Create a new tag for all
collideable objects. Click the + sign on the tag pane and rename the new tag
''Collidable.'' Then drag and drop the following actors into the Collidable tag:
 Projectile
 Target
 Soft Block
 Hard Block
 Wall
The result is shown in Figure 7-1.
Figure 7-1. The Collidable-tagged actors
Open the Target actor in the Actor Editor. Drag and drop a Collide behavior and
change the settings to Bounce when colliding with ''actor with tag'' ''Collidable,''
as shown in Figure 7-2.
Figure 7-2. Collide behavior
Repeat the same for actors Soft Block and Hard Block.
Ground Them on Earth: Gravity
As you are implementing gravity through acceleration, you will need to define an
Acceleration behavior.
Open the Target actor in the Actor Editor. Drag and drop an Acceleration
behavior and change the settings to:
 Direction: 270
 Relative to: scene
 Acceleration: 200