Connect Physics Core 2D API objects with joints

Note: This documentation is about writing C# scripts using the Unity.U2D.Physics API. To use 2D physics in the Unity Editor using components like the Rigidbody 2D component, refer to 2D physics instead.

To create a connection or constraint between two Physics Core 2D API objects, create a joint that connects the two physics bodies.

For example, create a fixed joint that welds two bodies together, or a distance joint that keeps them a set distance apart. Use joints to simulate real-world mechanical behaviours like a spring or a hinge.

All joints allow you to set the anchor points on the bodies they connect, and limits on the force and torque they apply. For more information, refer to the PhysicsJoint API.

Follow these steps:

Create two PhysicsBody objects. For more information, refer to Create a physics object.
Create a definition object for the type of joint you want. For example, a PhysicsDistanceJointDefinition object. Make it a public field so it displays its properties in the Inspector window.

For example:
```
public PhysicsDistanceJointDefinition jointDefinition = new PhysicsDistanceJointDefinition
{
    // Specify the two bodies to connect
    bodyA = body1,
    bodyB = body2,

    // Set the distance
    distance = 2f
};
```
A new definition has a set of default values. For more information about definitions and changing the default values, refer to Configure 2D physics properties using a definition.

For the full list of joint types, refer to PhysicsJoint.JointType.
To create the joint, use the CreateJoint method of your world object, and pass in the definition. For example:
```
PhysicsJoint distanceJoint = world.CreateJoint(jointDefinition);
```
If your script is in a MonoBehaviour class attached to a GameObject, adjust the properties in the Inspector window.

To configure the properties in your script instead, set the properties of the definition object before you create the joint. For more information, refer to Configure 2D physics properties using a definition.

If you destroy a body, any joints connected to that body are also destroyed. For more information, refer to Destroy physics objects and manage memory.

Example

The following example uses a fixed joint to create a fixed point that a large circle swings from.

using UnityEngine;
using Unity.U2D.Physics; 

public class CreateJoint : MonoBehaviour
{
    public PhysicsDistanceJointDefinition jointDefinition = new PhysicsDistanceJointDefinition();

    void Awake()
    {
        PhysicsWorld world = PhysicsWorld.defaultWorld;

        // Create a static fixed body
        PhysicsBodyDefinition bodyDefinition1 = new PhysicsBodyDefinition{
            type = PhysicsBody.BodyType.Static,
            position = new Vector2(-5f, 0f),
        };
        PhysicsBody object1 = world.CreateBody(bodyDefinition1);

        // Create a large circle below the fixed body
        PhysicsBodyDefinition bodyDefinition2 = new PhysicsBodyDefinition{
            type = PhysicsBody.BodyType.Dynamic,
            position = new Vector2(5f, 0f),
        };
        PhysicsBody object2 = world.CreateBody(bodyDefinition2);
        object2.CreateShape(new CircleGeometry { radius = 3f });

        // Add the bodies to the joint definition
        jointDefinition.bodyA = object1;
        jointDefinition.bodyB = object2;
        
        // Add the joint to the world
        world.CreateJoint(jointDefinition);        
    }
}

Additional resources

Combine Physics Core 2D API shapes

Destroy Physics Core 2D API objects and manage memory

Unity Manual

Connect Physics Core 2D API objects with joints

Example

Additional resources