def METHOD_NAME(backend_class):
    """
    Test that the correct velocity attribute is selected by the mapper.
    `PySDM.attributes.physics.relative_fall_velocity.RelativeFallVelocity`
    should only be selected when `PySDM.dynamics.RelaxedVelocity` dynamic exists.
    """
    builder_no_relax = Builder(n_sd=1, backend=backend_class())
    builder_no_relax.set_environment(Box(dt=1, dv=1))
    builder_no_relax.request_attribute("relative fall velocity")
    # with no RelaxedVelocity, the builder should use TerminalVelocity
    assert isinstance(
        builder_no_relax.req_attr["relative fall velocity"], TerminalVelocity
    )
    builder = Builder(n_sd=1, backend=backend_class())
    builder.set_environment(Box(dt=1, dv=1))
    builder.add_dynamic(RelaxedVelocity(tau=1))
    builder.request_attribute("relative fall velocity")
    # with RelaxedVelocity, the builder should use RelativeFallVelocity
    assert isinstance(builder.req_attr["relative fall velocity"], RelativeFallVelocity)
    # requesting momentum with no dynamic issues a warning
    builder = Builder(n_sd=1, backend=backend_class())
    builder.set_environment(Box(dt=1, dv=1))
    with pytest.warns(UserWarning):
        builder.request_attribute("relative fall momentum")