To help the programmer

Using the physical property of gravity, you can specify the object of attraction to the earth. AffectedByGravity – boolean value that indicates whether this physics body is affected by the physics world’s gravity.
The physics world’s gravity property defines the gravitational forces applied to volume-based bodies in the scene. The default value is true. This property is ignored on edge-based bodies, which are already unaffected by gravity.
Physics bodies with affectedByGravity set to false are still affected by the gravity fields created by linearGravityField (withVector) and radialGravityField ().

Swift Programming Language