In deriving the equation of motion for the motor, the friction on the shaft of the motor is approximated as a linear function of the shaft velocity. The approximation that the friction on the shaft of the motor K ,is a linear function of the shaft velocity is made. Newton's law of motion states that the sum of all force applied on the shaft is linearly related to the acceleration of the shaft multiplied by its moment of inertial I . Therefore, the preceding statement can be written as,