# FAQ: In Exercise 2.10, what do the variables phi represent?

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(Contributed by Richard Murray, 1 October 2012)

The variables and are the angles of the two masses on the shaft. The velocity of these masses are given by . The remaining parameter values describe the physical constants for the system: represents moments of inertia, is the damping coefficient, represents the shaft stiffness, is the torque constant for the motor, and is the disturbance torque applied at the end of the shaft.