code: public void addDirection(double extrp, double fh, double fv)
      {
          fhLast = fh;
          fvLast = fv;
          this.fh += fh * extrp;
          this.fv += fv * extrp;
          fixForce();
      }
docs: Increase forces with input value.
  
  @param extrp The extrapolation value.
  @param fh The added horizontal force.
  @param fv The added vertical force.