Notation

$y$

• Distance of the location/cut of interest from equilibrium position
• $y_s$ is the static $y$
• $y_d$ is the dynamic $y$
• $\hat{y}$ is the displacement $y$
  • Measured from static equilibrium position
  • $y=y_s+\hat{y}$

$d$

• Distance of the body/system’s movement from equilibrium position
• $y_R$ is the relative displacement $y$
  • Measured from a given distance $d$
  • $y_R=y-d\to y=y_R+d$