# IMPACT

The IMPACT function models impact forces acting on bodies during collision. The elastic properties of the boundary surface between the two bodies can be tuned as desired.

## Format

$\text{Impact}\left(x,\stackrel{˙}{x},{x}_{1},k,e,{c}_{\mathrm{max}},d\right)$

## Arguments

$x$
The independent variable. For example, to use the z-displacement of I marker with respect to J marker as resolved in the reference frame of RM marker as the independent variable, specify $x$ as DZ({marker_i.idstring}, {marker_j.idstring}, {marker_rm.idstring}).
$\stackrel{˙}{x}$
The time derivative of the independent variable. For example, if $x$ is specified as above, then $\stackrel{˙}{x}$ will be VZ({marker_i.idstring}, {marker_j. idstring}, {marker_rm.idstring}).
${x}_{1}$
The lower bound of $x$ . If $x$ is less than ${x}_{1}$ , the impact function returns a positive value, otherwise it returns zero.
$k$
The stiffness of the boundary surface interaction. It must be non-negative.
$e$
The exponent of the force deformation characteristic. For a stiffening spring characteristic, $e$ must be greater than 1.0 and for a softening spring characteristic, $e$ must be less than 1.0. It must always be positive.
${c}_{\mathrm{max}}$
The maximum damping coefficient. It must be non-negative.
$d$
The penetration at which the full damping coefficient is applied. It must be positive.

(1)

## Example

<Reference_Variable
id                    = "30300700"
type                  = "EXPRESSION"
expr                  = "IMPACT(DZ(10301030,30302030),
VZ(10301030,30302030),2.5,2500,1,1000,2.5)"
/>
OR
<Force_Vector_TwoBody
id                    = "10501"
type                  = "ForceOnly"
i_marker_id           = "10515721"
j_floating_marker_id  = "10516722"
ref_marker_id         = "10516720"
fx_expression         = "0"
fy_expression         = "0"
fz_expression         = "IMPACT(DZ(10515720,10516720,10515720),
VZ(10515720,10516720,10515720),1,10000.0,1.0,0.5,0.1)"
/>