如何在Drake中创建一个虚拟的无质量的身体？

import numpy as np
from pydrake.math import RigidTransform, RollPitchYaw
from pydrake.multibody.tree import SpatialInertia, RigidBody, RigidBodyFrame

# 创建一个刚体
body = RigidBody()

# 创建一个刚体的框架
frame = RigidBodyFrame(body, RigidTransform())

# 设置刚体的质量和惯量
inertia = SpatialInertia(
    mass=0.0,  # 设置质量为0，即无质量
    com=np.zeros(3),  # 设置质心位置
    rotational_inertia=np.zeros((3, 3))  # 设置惯量矩阵
)

# 设置刚体的形状
body.add_geometry(frame, inertia)

from pydrake.all import DiagramBuilder, Simulator

# 创建一个仿真器
builder = DiagramBuilder()
simulator = Simulator(builder.Build())

# 将刚体添加到仿真器中
context = simulator.get_mutable_context()
context.get_mutable_continuous_state().get_mutable_generalized_position().set_value(np.zeros(6))  # 设置初始位置和姿态
context.get_mutable_continuous_state().get_mutable_generalized_velocity().set_value(np.zeros(6))  # 设置初始速度

# 运行仿真
simulator.StepTo(1.0)  # 模拟1秒的时间