Why the damping exist in spite of I didn't set anything up?

[vino-volare]

I'm lerning the bevy and rapier for make vehicle simulator, and I'm trying rapier2d right now for a simple learning experience.

I'll generate one spring and mass for each for simple harmonic motion, so I tried this code.
Simply, Two rigid bodies are exist and each is connected with a prismatic joint.
The joint motor with stiffness and zero damping force applied to the prismatic joint.

use bevy::prelude::*;
use bevy_rapier2d::prelude::*;
fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_plugins(RapierPhysicsPlugin::<NoUserData>::pixels_per_meter(100.0))
        .insert_resource(RapierConfiguration {
            timestep_mode: TimestepMode::Fixed { dt: 0.01, substeps: 1 },
            ..Default::default()
        })
        .add_plugins(RapierDebugRenderPlugin::default())
        .add_systems(Startup, setup_graphics)
        .add_systems(Startup, setup_physics)
        .insert_resource(Time::<Fixed>::from_seconds(0.01))
        .run();
}

fn setup_graphics(mut commands: Commands) {
    commands.spawn(Camera2dBundle::default());
}

fn setup_physics(mut commands: Commands) {

    let fixed_box = commands.spawn((
            TransformBundle::from(Transform::from_xyz(0.0, 100.0, 0.0)),
            RigidBody::Fixed,
        ))
        .id();


    let arm = commands.spawn((
            TransformBundle::from(Transform::from_xyz(0.0, 80.0, 0.0)),
            RigidBody::Dynamic,
            Collider::cuboid(30.0, 30.0),
        ))
        .id();


    let joint = PrismaticJointBuilder::new(Vec2::new(0.0, 1.0))
        .local_anchor1(Vec2::new(0.0, 0.0))
        .local_anchor2(Vec2::new(0.0, 20.0))
        .motor_position(-5.0, 500.0, 0.0);

    commands.entity(arm).with_children(|cmd| {
        cmd.spawn(ImpulseJoint::new(fixed_box, joint));
    });
}

I expected that motion would not damp.
But unfortunately I observed that is damping in spite of I didn't set anything up.

Also, The same phenomenon also occurred with a pendulum using revolute joint.

How to fix it up?

[sebcrozet]

Hi! Every constraint in Rapier is modeled as implicit springs. Implicit springs a great for stability, but introduce numerical damping that can’t really be fixed externally except by reducing the timestep length, or increasing the number of solver substeps.

[vino-volare]

Thanks for answering! I understood that. ;)