Suppose now that we wish to drive the mass along a frictionless surface using a variable force . This is similar to the previous example, except that we now want the traveling-wave components of the force on the mass to sum to instead of 0 :
Since and are given, must be computed as . This is shown in Fig.F.9.
The simplified form in Fig.F.9b can be interpreted as a wave digital spring with applied force delivered from an infinite source impedance. That is, when the applied force goes to zero, the termination remains rigid at the current displacement.