Two things are required for an object to use the physics engine built into Unity3D:
The rigid body allows your objects to react to gravity, receive forces, and receive torque. The rigid body is the component that makes your game object interact and move in a realistic world with other rigid bodies.
The collider is used to send and receive collision information between objects. Rigid bodies are typically, but not always, used in conjunction with primitive (sphere, box, capsule) colliders.
Managing big immovable objects, such as a grandstand or a tree, is very simple in Unity3D. Simply attach a primitive collider component to the object, and our player can run into them all day long and they will ...