2D Joints

2D joints are the 2D counterparts of the 3D 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 physics components, and are made to work exclusively with 2D GameObjectsThe fundamental object in Unity scenes, which can represent characters, props, scenery, cameras, waypoints, and more. A GameObject’s functionality is defined by the Components attached to it. More info
See in Glossary. You can only attach 2D joints to GameObjects which have an attached RigidbodyA component that allows a GameObject to be affected by simulated gravity and other forces. More info
See in Glossary 2D component, or to a fixed position in world space. You can tell a 2D joint from its 3D counterpart in the Component browser by its name ending in ‘2D’.

There are many different types of 2D joints available. Refer to each joint’s respective page for detailed information about their properties and uses.

• Distance Joint 2D: This joint attaches two GameObjects controlled by Rigidbody physics together while keeping them a set distance apart.
  
• Fixed Joint 2DA 2D joint type which is completely constrained, allowing two objects to be held together. Implemented as a spring so some small motion may still occur. More info
  See in Glossary: This joint keeps two objects in a certain position relative to each other, so that the objects are always offset at a set position and angle. This is useful for objects that need to react as if they are rigidly connected. You can also use this joint to create a less rigid connection with some flexibility.
  
• Friction Joint 2D: This joint reduces both the linear and angular velocities between two GameObjects controlled by Rigidbody physics to zero (i.e. it slows them down and stops them). For example, use this joint to create a rotating platform that still resists the rotation.
  
• Hinge Joint 2D: This joint allows a GameObject controlled by Rigidbody physics to be attached to a point in space around which it can rotate. For example, use this joint as the pivot on a pair of scissors.
  
• Relative Joint 2DA 2D joint that allows two game objects controlled by Rigidbody physics to maintain in a position based on each other’s location. Use this joint to keep two objects offset from each other, at a position and angle you decide More info
  See in Glossary: This joint allows two GameObjects controlled by Rigidbody physics to maintain a position based on each other’s location. Use this joint to keep two objects at a certain offset from each other. For example, use this joint to attach two additional guns to a spaceship which follows its position.
  
• Slider Joint 2D: This joint allows a GameObject controlled by Rigidbody physics to slide along a line in space. For example, use this joint to create a sliding door object.
  
• Spring Joint 2D: This joint allows two GameObjects controlled by Rigidbody physics to react as if they were attached together by a spring.
  
• Target Joint 2D: This joint connects to a specified target, rather than another Rigidbody object, as other joints do. It is a spring type joint, which you could use for picking up and moving an object acting under gravity, for example.
  
• Wheel Joint 2D: This joint simulates the behavior of a rolling wheel, with a suspension ‘spring’ maintaining its distance from the main body of the ‘vehicle’.

Constraints

A constraint is a ‘rule’ which a joint will try to ensure isn’t permanently broken. The are different types of constraints, and all joints provide one or more constraints that apply to and governs the Rigidbody 2D behavior. Some constraints limit behavior such as ensuring a Rigidbody stays on a line, or in a certain position. Some are ‘driving’ constraints which actively compel a Rigidbody object to behave in a certain way, such as trying to make an object maintain a certain speed.

Temporarily breaking constraints

The physics system expects that constraints can be temporarily broken, such as when the objects move further apart than their distance constraint tells them to, or objects may move faster than what their speed constraint is set to. When a constraint isn’t broken, the joint doesn’t apply any forces and does little work. It is when a constraint is broken that the joint applies forces to fix the constraint.

For example, for the ‘driving’ constraints mentioned above, the joint applies forces to maintain the distance or ensures the speed set by the constraint. While this application of force is usually performed quickly, it does not always instantly fix the constraint and instead it fixes the constraint gradually over time. This can lead to joints appearing to stretch or appear less rigid. The lag happens because the physics system is trying to apply joint-forces to fix constraints, while at the same time other physics forces are still acting to break those same constraints. In addition to the conflicting forces acting on GameObjects, some joints are more stable and react faster than others.

Whatever constraints the joint provides, the joint only uses forces to fix the constraint. These are either a linear (straight line) force or angular (torque) force.

Tip: It is recommended to be cautious when applying large forces to Rigidbody objects that have joints attached, especially those with large masses, due to the conflicting forces acting on joints.

Permanently breaking joints

All joints are able to monitor the force or torque that they are applying to stay within its own constraints. Some joints monitor both force and torque while others monitor only force. This allows you to know when a joint exceeds a specific force and/or torque in trying to maintain its constraints, and you can specify these thresholds as Joint2D.breakForce and Joint2D.breakTorque. When a joint exceeds these thresholds, it is known as “joint breaking”.

You can specify the action to be taken when a joint “breaks” with Joint2D.breakAction. The default break action is to destroy the Joint2D component, and you can refer to JointBreakAction2D for other available fixed actions.