> ## Documentation Index
> Fetch the complete documentation index at: https://dadd.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Model Mutation

> Modify physics parameters at runtime for domain randomization

Methods for modifying model parameters at runtime. Useful for domain randomization in RL training.

## setGravity(g)

Change the gravity vector.

```tsx theme={null}
api.setGravity([0, 0, -9.81]);    // Earth gravity (Z-up)
api.setGravity([0, 0, -1.62]);    // Moon gravity
api.setGravity([0, 0, 0]);        // Zero gravity
```

<ParamField body="g" type="[number, number, number]" required>
  Gravity vector in m/s^2.
</ParamField>

## setTimestep(dt)

Change the simulation timestep.

```tsx theme={null}
api.setTimestep(0.001);  // 1ms (more stable, slower)
api.setTimestep(0.005);  // 5ms (faster, less stable)
```

<ParamField body="dt" type="number" required>
  Timestep in seconds.
</ParamField>

<Warning>
  Changing the timestep affects simulation stability. Smaller values are more stable but slower. Large values may cause the simulation to explode.
</Warning>

## setBodyMass(name, mass)

Change a body's mass.

```tsx theme={null}
api.setBodyMass("block", 0.5);   // 500g
api.setBodyMass("block", 2.0);   // 2kg
```

<ParamField body="name" type="string" required>
  Body name.
</ParamField>

<ParamField body="mass" type="number" required>
  New mass in kg.
</ParamField>

## setGeomFriction(name, friction)

Change a geom's friction coefficients.

```tsx theme={null}
api.setGeomFriction("floor", [1.0, 0.005, 0.0001]);  // High friction
api.setGeomFriction("ice", [0.1, 0.001, 0.00001]);    // Low friction
```

<ParamField body="name" type="string" required>
  Geom name.
</ParamField>

<ParamField body="friction" type="[number, number, number]" required>
  MuJoCo friction coefficients: `[slide, torsional, rolling]`.
</ParamField>

## setGeomSize(name, size)

Change a geom's size.

```tsx theme={null}
api.setGeomSize("box", [0.05, 0.05, 0.05]);  // 10cm cube
api.setGeomSize("ball", [0.03, 0, 0]);        // 3cm radius sphere
```

<ParamField body="name" type="string" required>
  Geom name.
</ParamField>

<ParamField body="size" type="[number, number, number]" required>
  New size (semantics depend on geom type — see [MuJoCo docs](https://mujoco.readthedocs.io/en/latest/XMLreference.html#body-geom)).
</ParamField>

## Example: Domain Randomization

```tsx theme={null}
function DomainRandomizer() {
  const { api } = useMujoco();

  function randomize() {
    // Randomize gravity
    const gz = -9.81 + (Math.random() - 0.5) * 2;
    api.setGravity([0, 0, gz]);

    // Randomize object mass
    api.setBodyMass("cube", 0.05 + Math.random() * 0.2);

    // Randomize floor friction
    const friction = 0.5 + Math.random() * 1.0;
    api.setGeomFriction("floor", [friction, 0.005, 0.0001]);

    // Reset to apply changes
    api.reset();
  }

  return <button onClick={randomize}>Randomize</button>;
}
```