# Force: Multi-Point

Model ElementForce_MultiPoint defines a force and torque acting between many Reference_Markers.

## Description

The force and the torque can only be linear functions of the relative displacement and velocity of the various Reference_Markers.

## Format

< Force_MultiPoint
id                = "integer"
ref_marker_id     = "integer"
kmtx_id           = "integer"
[{ cmtx_id           = "integer"
|    cratio            = "real"
}]
[ preload_mtx_id    = "integer" ]
[ length_mtx_id     = "integer" ]
n_i_marker        = "integer"  >
integer1   integer1   … integerN
</Force_MultiPoint >

## Attributes

id
Element identification number (integer>0). This number is unique among all Force_MultiPoint elements and uniquely identifies the element.
ref_marker_id
Specifies the Reference_Marker which serves as the reference frame in which relative velocities are computed. The components of the relative displacement, relative velocity and force vectors are resolved in the coordinate system specified by ref_marker_id.
kmtx_id
Specifies the Reference_Matrix element which contains the stiffness matrix. This is a 6N*6N matrix, where N is the number of I Reference_Markers.
This is a required parameter.
cmtx_id
Specifies the Reference_Matrix element which contains the damping matrix. This is a 6N*6N matrix, where N is the number of I Reference_Markers.
This parameter is optional. Default is a zero matrix.
cratio
Defines a damping scale factor. If this is specified, the damping matrix is calculated as C = cratio * K.
This parameter is optional. Default is zero.
Specifies the 6N*1 Reference_Matrix element which contains the preload values for each pair of markers between which the Force_MultiPoint is applied. N is the number of I Reference_Markers.
This parameter is optional. Default is a zero matrix.
length_mtx_id
Specifies the 3N*1 Reference_Matrix element which contains the reference length values for each pair of markers between which the Force_MultiPoint is applied. N is the number of I Reference_Markers.
This parameter is optional. When omitted, the I Reference_Markers are taken to be at their reference locations.
n_i_marker
Specifies the number of I Reference_Marker elements between which the Force_MultiPoint is applied. If there are N such markers, then N integers specifying their IDs must follow.

## Example

The following is an implementation of a Force_MultiPoint element between three markers with ids 101 through 103. The reference marker has an id of 10. The stiffness matrix is specified using the Reference_Matrix with id 11. The damping ratio is specified as 0.05. The preload and length matrices are not specified.

<Force_MultiPoint
id                  = "1"
ref_marker_id       = "10"
kmtx_id             = "11"
cratio              = "0.05"
n_i_marker          = "3">
101 102 103
</Force_MultiPoint>