# Collision Physics Each separable part of a Fragment must have an associated collision object in order for physics to work. This means each bone with `Use Physics` enabled should also have a collision object linked to it. ### Rigging Collisions You link collisions to bones the same way you link Drawable Models to bones as described in [Drawables (.ydr) > Rigging](https://docs.sollumz.org/drawables.ydr/rigging#linking-bones-to-drawable-models).

Collision object linked to bone via a Child Of constraint

### Setting Mass Each collision object under a Fragment also has a Physics subpanel where you can define its mass.

#### Auto Mass Tool There is also a tool for automatically calculating mass based on the collision material's density and the object's volume. Navigate to `Sollumz Tools > Fragment > Set Mass`. There you will find the `Calculate Collision Mass tool`.

In order for this tool to work, the collision object must have a collision material added. ### Volume and Inertia Volume and Inertia are essential physics properties for collisions. You can find these options under `Object Properties > Sollumz`.

Typically when creating completely custom collisions, you would auto-calculate these values during export, as calculating these involves a lot of math relating to the dimensions and shape of your collisions.