Question

A system of two wheels fixed to each other is free to rotate about a friction less axis through the common center of the wheels and perpendicular to the page. Four forces are exerted tangentially to the rims of the wheels, as shown. The magnitude of the net torque on the system about the axis is 

(A) zero 

(B) FR 

(C) 2FR 

(D) 5FR 

(E) 14FR

Answer 1

Correct option (C) 2FR

Find the torques of each using proper signs and add up. 

+ (1) – (2) + (3) + (4) 

 +F(3R) – (2F)(3R) + F(2R) +F(3R) = 2FR