# Rigid bodies

A rigid body is the default kind of entity in Genesis World: a solid that does not deform, simulated by the rigid solver. It is what you get from {doc}`Hello, Genesis World </user_guide/getting_started/hello_genesis>`, and it covers most of robotics, robot arms, grippers, mobile bases, and the props they interact with. This page is the overview of that entity type; the rest of this section covers the non-rigid families that build on the same `Scene`.

## Adding a rigid body

An entity is rigid whenever its `material` is `gs.materials.Rigid`, which is the default, so you usually pass only a {doc}`morph </user_guide/assets/loading_assets>`:

```python
box = scene.add_entity(gs.morphs.Box(pos=(0, 0, 0.5), size=(0.2, 0.2, 0.2)))
franka = scene.add_entity(gs.morphs.MJCF(file="xml/franka_emika_panda/panda.xml"))
```

Both are rigid entities. The box is a single body; the Franka is **articulated**, a tree of rigid **links** connected by **joints**.

## Single bodies and articulated bodies

Every rigid entity is a `RigidEntity`, and you interact with it through its own methods rather than a global handle. An articulated entity exposes its structure:

- **Links** (`entity.links`, `entity.n_links`): the individual rigid bodies in the tree.
- **Joints** (`entity.joints`, `entity.n_joints`): the connections between links.
- **Degrees of freedom** (`entity.n_dofs`): the independent coordinates the joints move along. A single free body has 6 dofs; a fixed box has none.

You read and write state through the entity: `get_pos()` and `get_quat()` for the base pose, and `get_dofs_position()` for joint positions. Driving those dofs with a controller is covered in {doc}`Control your robot </user_guide/getting_started/control_your_robot>` and, for arms, {doc}`Robot control </user_guide/robot_control/inverse_kinematics_motion_planning>`.

## Fixed and free bases

Whether an entity's base is bolted to the world or floats freely depends on the morph. An MJCF file specifies the base joint itself; a URDF base is free (a 6-dof joint to the world) unless you pass `fixed=True`. See {doc}`Loading assets </user_guide/assets/loading_assets>` for the details.

## Physical properties

The rigid material sets how a body interacts physically. Pass a configured `gs.materials.Rigid` to override the defaults:

```python
box = scene.add_entity(
    gs.morphs.Box(pos=(0, 0, 0.5), size=(0.2, 0.2, 0.2)),
    material=gs.materials.Rigid(
        rho=1000.0,     # density in kg/m³, used to estimate mass
        friction=1.0,   # surface friction coefficient
    ),
)
```

Contact, collision geometry, and constraints, how these bodies actually push on each other, are governed by the rigid solver and documented under {doc}`Theory and modelling </user_guide/theory/rigid_collision/index>`.

## See also

- {doc}`/user_guide/getting_started/control_your_robot`: driving a rigid robot with the built-in controller.
- {doc}`/user_guide/robot_control/inverse_kinematics_motion_planning`: inverse kinematics and collision-free planning.
- {doc}`beyond_rigid_bodies`: the non-rigid solvers (MPM, FEM, PBD, SPH) that share the scene.
- {doc}`/user_guide/theory/rigid_collision/index`: the contact and constraint model behind rigid dynamics.