Spring Joint 2D fundamentals

This jointA physics component allowing a dynamic connection between Rigidbody components, usually allowing some degree of movement such as a hinge. More info
See in Glossary behaves like a spring, while keeping a linear distance between two points. You set this via the Distance setting. Those two points can be two Rigidbody2D components or a Rigidbody2D component and a fixed position in the world. (Connect to a fixed position in the world by setting Connected Rigidbody to None). The joint applies a linear force to both rigid bodies. It doesn’t apply torque (an angle force).

The joint uses a simulated spring. You can set the spring’s stiffness and movement:

A stiff, barely moving spring…

• A high (1,000,000 is the highest) Frequency == a stiff spring.

• A high (1 is the highest) Damping RatioA joint setting to control spring oscillation. A higher damping ratio means the spring will come to rest faster. More info
  See in Glossary == a barely moving spring.

A loose, moving spring…

• A low Frequency == a loose spring.

• A low Damping Ratio == a moving spring.

When the spring applies its force between the objects, it tends to overshoot the distance you have set between them, and then rebound repeatedly, giving in a continuous oscillation. The Damping Ratio sets how quickly the objects stop moving. The Frequency sets how quickly the objects oscillate either side of the target distance.

This joint has one constraint:

• Maintain a zero linear distance between two anchor points on two RigidbodyA component that allows a GameObject to be affected by simulated gravity and other forces. More info
  See in Glossary objects.

You can use this joint to construct physical objects that need to react as if they are connected together using a spring or a connection which allows rotation. For example:

• A character who’s body is composed of multiple objects that act as if they are semi-rigid. Use the Spring JointA joint type that connects two Rigidbody components together but allows the distance between them to change as though they were connected by a spring. More info
  See in Glossary to connect the character’s body parts together, allowing them to flex to and from each other. You can specify whether the body parts are held together loosely or tightly.

Note: Spring Joint 2D uses a Box 2D spring-joint, which the Distance Joint 2D also uses with its frequency set to zero.

Additional resources

• Joints 2D
• Spring Joint 2D component reference