====== Entering Inertial Values Using Expressions ======

It is possible to include any regression equations for the inertia and center of mass because Visual3D allows the user to put [[visual3d:documentation:pipeline:expressions:expressions_overview|expressions]] into the edit boxes.

===== Example: Typical Right Thigh Segment =====

Consider a typical right thigh segment, which is populated with the default inertial values.

{{:InertiaExpression1.jpg}}

If the user selects this segment to be a CUSTOM_SEG, the edit boxes can be edited.

{{:InertiaExpression2.jpg}}

===== Example: Frustra of Right Cones =====

In this example we consider a conical segment:

{{:frustraofrightcones1.gif}}

We will create an expression for the Axial component of the center of mass based on the default computations described [[visual3d:documentation:modeling:segments:segment_inertia#inertial_properties_of_a_cone_conical_frustrum|elsewhere in the documentation]].

{{:frustraofrightcones2.png}}

For convenience create a new Subject Metric X and enter the following expression:

<code>
RTH_DISTAL_RADIUS/RTH_PROXIMAL_RADIUS:
</code>

{{:InertiaExpression3.jpg}}

We can then use the Subject Data Metric X that we just created in another expression. Enter the following expression into the Segment Properties Axial edit box:

<code>
(1+2*X+3*X^2)/4/(1+X+X^2)*RTH_SEG_LENGTH
</code>

{{:InertiaExpression4.jpg}}

**Note**: the edit box doesn't show the entire expression, which is a nuisance