collider_mass_properties

The mass properties of a rigid-body is computed as the sum of the mass-properties manually set by the user for the rigid-body, plus the mass-properties of the colliders attached to it. There are two ways to define the mass-properties of a collider:

The easiest, automatic, way: by giving the collider a non-zero density (the default density is 1.0) or a non-zero mass. This will make sure the other mass-properties like the angular inertia tensor are computed automatically from the collider's shape.
The manual way: by giving an explicit mass and angular inertia to the collider.

It is recommended to use the density-based or mass-based approaches as it will ensure the automatically-computed mass-properties are coherent with the geometric shape. Wrong mass-properties (especially the angular inertia part and center-of-mass location) may lead to odd behaviors. The manual approach is usually useful when modeling real-world objects for which you already know the real-world mass, center-of-mass, and angular inertia tensor.

The mass-properties of a collider can be set when the collider is created, with ColliderBuilder::density, ColliderBuilder::mass, or ColliderBuilder::mass_properties:

Example 2D
Example 3D

let rigid_body = RigidBodyBuilder::dynamic().build();
let rigid_body_handle = world.insert_body(rigid_body);
// First option: by setting the density of the collider (or we could just leave
//               its default value 1.0).
let collider = ColliderBuilder::cuboid(1.0, 2.0).density(2.0).build();
// Second option: by setting the mass of the collider.
let collider = ColliderBuilder::cuboid(1.0, 2.0).mass(0.8).build();
// Third option: by setting the mass-properties explicitly.
let collider = ColliderBuilder::cuboid(1.0, 2.0)
    .mass_properties(MassProperties::new(Vector::new(0.0, 1.0), 0.5, 0.3))
    .build();
// When the collider is attached, the rigid-body's mass and angular
// inertia is automatically updated to take the collider into account.
world.insert_collider(collider, Some(rigid_body_handle));

let rigid_body = RigidBodyBuilder::dynamic().build();
let rigid_body_handle = world.insert_body(rigid_body);
// First option: by setting the density of the collider (or we could just leave
//               its default value 1.0).
let collider = ColliderBuilder::cuboid(1.0, 2.0, 3.0).density(2.0).build();
// Second option: by setting the mass of the collider.
let collider = ColliderBuilder::cuboid(1.0, 2.0, 3.0).mass(0.8).build();
// Third option: by setting the mass-properties explicitly.
let collider = ColliderBuilder::cuboid(1.0, 2.0, 3.0)
    .mass_properties(MassProperties::new(
        Vector::new(0.0, 1.0, 0.0),
        0.5,
        Vector::new(0.3, 0.2, 0.1),
    ))
    .build();
// When the collider is attached, the rigid-body's mass and angular
// inertia is automatically updated to take the collider into account.
let collider_handle =
    world.insert_collider(collider, Some(rigid_body_handle));

They can also be modified after the creation of the collider with Collider::set_density, Collider::set_mass, or Collider::set_mass_properties. Each of these methods (as well as the builder methods above) replaces the mass-properties previously set by any of the others: for example, calling Collider::set_mass on a collider built with ColliderBuilder::density makes its density be ignored. The mass-properties of the rigid-body the collider is attached to are then updated automatically at the next simulation step (or immediately with RigidBody::recompute_mass_properties_from_colliders).