# 3.1.11: Point-Mass

Karamba3D is capable of calculating the natural vibration modes and frequencies of structures (see section [3.5.7](../3.5-algorithms/3.5.7-natural-vibrations)). For results to match reality the inertia properties of a structure need to be modelled correctly. Masses of elements (e.g. beams, trusses, shells) are automatically taken care of. All other items need to be included via point-masses.

{% hint style="info" %}
Be aware of the fact that masses defined with the **“Point-Mass”**-component do not have a weight but inertia only! They also do not add to the mass of a model as output at the **"Assemble"**-component.
{% endhint %}

Thus they effect only the calculation of natural frequencies. The “Point-Mass” component expects a mass in kilogram at its input-plug **“Mass”** (see fig. 3.1.11.1). Nodes where masses shall sit can be identified by supplying node indexes or positions (just like for point-loads). Point masses get displayed as green spheres. Their diameters result from the volume calculated as mass divided by density. The latter defaults to $$7850kg/m^3$$ (steel) and can be provided at the input-plug-**“rho”**.

![Fig. 3.1.11.1: Vibration mode of beam with point mass in the middle](https://442610158-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MCkDlhlZpqUmakSqOrp%2Fsync%2Fb553e72f06c045efbb9d4e90724201da170c1e0c.png?generation=1595316047976473\&alt=media)



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