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(sec-item-markernodeposition)=
# MarkerNodePosition

A node-Marker attached to a position-based node. It can be used for connectors, joints or loads where position is required. If connectors also require orientation information, use a MarkerNodeRigid.

## Interface

- Provides: `Position`
- Nodes it can be attached to: [](#sec-item-nodepoint), [](#sec-item-nodepoint2d), [](#sec-item-noderigidbodyep), [](#sec-item-noderigidbodyrxyz), [](#sec-item-noderigidbodyrotveclg), [](#sec-item-noderigidbody2d), [](#sec-item-nodepoint2dslope1), [](#sec-item-nodepointslope1), [](#sec-item-nodepointslope12), [](#sec-item-nodepointslope23), [](#sec-item-nodepointground)
- Connectors, constraints and loads that can use it: [](#sec-item-objectconnectorspringdamper), [](#sec-item-objectconnectorcartesianspringdamper), [](#sec-item-objectconnectorgravity), [](#sec-item-objectconnectorhydraulicactuatorsimple), [](#sec-item-objectconnectordistance), [](#sec-item-objectcontactspheresphere), [](#sec-item-objectcontactspheretriangle), [](#sec-item-objectjointspherical), [](#sec-item-objectjointrevolute2d), [](#sec-item-loadforcevector)

## Parameters

The parameters of the item; in a dictionary, its type is 'NodePosition':

| name | type | size | default | description |
|---|---|---|---|---|
| **name** | String |  | '' | marker's unique name |
| **nodeNumber** | NodeIndex |  | invalid (-1) | node number to which marker is attached to |
| **visualization** | VMarkerNodePosition |  |  | parameters for visualization of item |

## Visualization parameters

The parameters of `VMarkerNodePosition`, given as `visualization`:

| name | type | size | default | description |
|---|---|---|---|---|
| **show** | Bool |  | True | set true, if item is shown in visualization and false if it is not shown |

## Drawing

Drawn as [all markers](#sec-drawing-markers).
(description-markernodeposition)=
## Detailed description

### Marker quantities

| quantity | symbol | as computed |
|---|---|---|
| position | $\LU{0}{\pv}_m$ | the position of the node, as its output variable `Position` |
| velocity | $\LU{0}{\vv}_m$ | the velocity of the node, as its output variable `Velocity` |

Both are global, or in the frame the object reads the node in (see the page of the node).

### Jacobians

The position Jacobian of the node, $\LU{0}{\Jm_{pos}} = \partial \LU{0}{\pv} / \partial \qv$ with
respect to the node's coordinates: the unit matrix for `NodePoint`, the first three columns for a
rigid body node, and for a 2D node the $x$ and $y$ rows.

(miniexample-markernodeposition)=
## Mini example


```python
#the position of a node: a mass hanging on a spring from a ground node, released at rest
nMass = mbs.AddNode(NodePoint(referenceCoordinates=[0,0,-1]))
mbs.AddObject(ObjectMassPoint(nodeNumber=nMass, mass=1))
mMass = mbs.AddMarker(MarkerNodePosition(nodeNumber=nMass))
mFixed = mbs.AddMarker(MarkerNodePosition(nodeNumber=nGround))
k = (2*np.pi)**2 #1 Hz
mbs.AddObject(ObjectConnectorSpringDamper(markerNumbers=[mFixed, mMass], stiffness=k, referenceLength=1))
mbs.AddLoad(LoadForceVector(markerNumber=mMass, loadVector=[0,0,-9.81]))

mbs.Assemble()
simulationSettings = exu.SimulationSettings()
simulationSettings.timeIntegration.endTime = 0.5 #half a period
mbs.SolveDynamic(simulationSettings)

#lowest point: twice the static deflection, -2*g/k
exu.sys['testResult'] = mbs.GetNodeOutput(nMass, exu.OutputVariableType.Displacement)[2] #-0.497
```


Examples (Ex) and TestModels (TM) that show this item, with weblink to github: [`doublePendulum2D.py`](https://github.com/jgerstmayr/EXUDYN/blob/master/python/Examples/doublePendulum2D.py) (Ex), [`pendulum2Dconstraint.py`](https://github.com/jgerstmayr/EXUDYN/blob/master/python/Examples/pendulum2Dconstraint.py) (Ex), [`interactiveTutorial.py`](https://github.com/jgerstmayr/EXUDYN/blob/master/python/Examples/interactiveTutorial.py) (Ex), [`simple4linkPendulumBing.py`](https://github.com/jgerstmayr/EXUDYN/blob/master/python/Examples/simple4linkPendulumBing.py) (Ex), [`connectorGravityTest.py`](https://github.com/jgerstmayr/EXUDYN/blob/master/python/TestModels/connectorGravityTest.py) (TM)