why the torque is negative in this case ? the question ask for determine the angle of twist of gear C with respect to B .
We have to 'fix' the B , and determine the twist of C ? So , imagine that we cut off the shaft and gear , now only left with gear B and gear C connected by a shaft . So , when B is fixed , the gear C rotate , with the direction of torques pointing to the left ( by using right hand grip rule ) , So , the finger point towards B , so it's negative ?