The aim of this joint is to maintain a relative linear and angular offset between two points. Those two points can be two Rigidbody 2D components or a Rigidbody 2D component and a fixed position in the world. (Connect to a fixed position in the world by setting Connected Rigidbody to None).
The linear and angular offsets are based upon the relative positions and orientations of the two connected points, so you change the offsets by moving the connected GameObjects in your Scene view.
The joint applies both linear and torque forces to connected Rigidbody 2D GameObjects. It uses a simulated spring that is pre-configured to be as stiff as the simulation can provide. You can change the spring’s value to make it weaker using the Frequency setting.
When the spring applies its force between the GameObjects, it tends to overshoot the desired distance between them and then rebound repeatedly, resulting in a continuous oscillation. The damping ratio determines how quickly the oscillation reduces and brings the GameObjects to rest. The frequency is the rate at which it oscillates either side of the target distance; the higher the frequency, the stiffer the spring.
Fixed Joint 2D has two simultaneous constraints:
You can use this joint to construct physical GameObjects that need to react as if they are rigidly connected. They can’t move away from each other, they can’t move closer together, and they can’t rotate with respect to each other, such as a bridge made of sections which hold rigidly together.
You can also use this joint to create a less rigid connection that flexes - for example, a bridge made of sections which are slightly flexible.
It is important to know the major differences between Fixed Joint 2D and Relative Joint 2D: