# 3.1.11: 点-质量 (Point-Mass)

Karamba3D能够计算结构的固有振动模式及频率（请参阅第[3.5.7](/chinese_1_3_3/3-in-depth-component-reference/3.5-algorithms/3.5.7-natural-vibrations.md)节）。为了使计算结果与实际情况相符，需对结构的惯性进行正确的模拟。元素（如梁、桁架、壳体等）的质量由程序自动处理。而所有其他项目都需要通过点-质量纳入计算。

{% hint style="info" %}
请注意，由**“Point-Mass（点-质量 ）”**运算器定义的质量没有重量，而只有惯性！不过，在“Assemble（模型构建）”运算器中，它们会作为输出添加至模型的质量中。
{% endhint %}

因此，它们仅影响自然频率的运算。在**“Point-Mass（点-质量 ）”**运算器上，**“Mass（质量）”**输入端输入值的单位为kg（如图3.1.11.1）。可通过提供节点索引或位置（类似点-荷载）来确定质量所在的节点。点质量的显示为绿色球体。其直径由质量除以密度得出的体积来计算的。后者的默认值为$$7850kg/m^3$$（钢），可通过**“rho”**输入端口来提供。

![图3.1.11.1：点-质量位于中间的梁的振动模式](/files/-M9XuT_i4asTdz_npyJW)


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