Answer
(a) The normal force applied to each side of the rotor is $30~N$
(b) $\tau = 5.76~N\cdot m$
Work Step by Step
(a) We can use Pascal's principle to find the normal force $F_2$ applied to each side of the rotor:
$\frac{F_2}{A_2} = \frac{F_1}{A_1}$
$F_2 = \frac{A_2~F_1}{A_1}$
$F_2 = \frac{(12.0~cm^2)~(7.5~N)}{3.0~cm^2}$
$F_2 = 30~N$
The normal force applied to each side of the rotor is $30~N$
(b) We can find the total torque on the rotor due to the two pads:
$\tau = 2\times~(r~F_2~\mu_k)$
$\tau = (2)(0.12~m)(30~N)(0.80)$
$\tau = 5.76~N\cdot m$
