Example: ``Piano hammer in flight''

Mass $m$ at constant velocity:

• State variable is in units of force ( $x(n) \isdef f^{{+}}(n-1)$)
• Physical force is $f(n)=f^{{+}}(n)+f^{{-}}(n)\equiv\message{CHANGE eqv TO equiv IN SOURCE}0$
• Nonzero state variable $\Rightarrow$ nonzero mass velocity