# RelativeStates

```model RelativeStates "Definition of relative state variables"
extends Translational.Interfaces.PartialTwoFlanges;

parameter StateSelect stateSelect = StateSelect.prefer "Priority to use the relative angle and relative speed as states";
SI.Position s_rel(start = 0, stateSelect = StateSelect.prefer) "Relative position used as state variable";
SI.Velocity v_rel(start = 0, stateSelect = StateSelect.prefer) "Relative velocity used as state variable";
SI.Acceleration a_rel(start = 0) "Relative angular acceleration";
equation
a_rel = der(v_rel);
s_rel = flange_b.s - flange_a.s;
v_rel = der(s_rel);
flange_a.f = 0;
flange_b.f = 0;

annotation (
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end RelativeStates;

```