# 3.1.9: Point-Mass

Karamba3D can calculate the natural vibration modes and frequencies of structures (see section [3.6.7](/3-in-depth-component-reference/3.5-algorithms/3.5.7-natural-vibrations.md)). To ensure accurate results, the inertia properties of a structure need to be correctly modeled. The masses of elements (e.g., beams, trusses, shells) are automatically included. All other items must be added via point masses.

{% hint style="info" %}
Note that masses defined with the **“Point-Mass”** component do not have weight but only inertia. They also do not contribute to the mass of the model as output by the **“Assemble”** component.
{% endhint %}

Therefore, they only affect the calculation of natural frequencies. The **“Point-Mass”** component expects a mass in kilograms at its **“Mass”** input plug (see Fig. 3.1.9.1). A vector supplied at **“SFacs”** specifies mass multipliers for the global X-, Y-, and Z-directions. In Fig. 3.1.9.1, the inertia of the mass acts only in the Z-direction. Nodes where masses are to be applied can be identified by supplying node indexes or positions (similar to point loads) to the **“Pos|Ind”** input plug. Point masses are displayed as green spheres, with diameters calculated from the volume as mass divided by density. The default density is for steel and can be specified at the **“rho”** input plug.

<figure><img src="/files/WZCAsCOxtkELh7EsUnou" alt=""><figcaption><p>Fig. 3.1.9.1: Vibration mode of beam with point mass in the middle</p></figcaption></figure>

{% file src="/files/ZJmujjfipbwiKCnXjfoo" %}


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