2D joints are the 2D counterparts of the 3D joint physics components, and are made to work only with 2D GameObjects. You can only attach 2D joints to GameObjects which have an attached Rigidbody 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.
A constraint is a ‘rule’ which a joint will try to ensure isn’t permanently broken. There 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.
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 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’s 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 doesn’t 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’s 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.
All joints are able to monitor the force or torque that they’re 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 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’s 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.