Solving Torsion Direction Homework Statement

[yecko]

Homework Statement

Homework Equations

Angle of twist = TL/JG

The Attempt at a Solution

T & L is inversely proportional.
But how can i start attempt the question?
I think it this way: like moment=L*F, and torsion torque is like force, angle of twist is similar to moment (as TL are also positively proportional to angle of twist)
For part (1), T at C would play a more important role to the part 12 for being nearer, and T at B would only contribute half as that at C. Both upper and lower end should havr 0.5T with direction same as C.
For part (2), both upper and lower end should have T and upper with same direction as C and lower part with direction same as D.

The model answer is the answer highlighted in res in the picture above. And I don't understand why. Can anyone explain to me? Thanks.

 

[scottdave]

They are not asking about angles. Look at the whole rod, to start. Does the fixed end A experience any torque? If so how much? Sum up all of the external torques. What should that sum be equal to in this situation?

 

[haruspex]

T at C would play a more important role to the part 12 for being nearer,

I think you are saying that a torque T applied, in isolation, at C would cause a greater strain over 12 than the same torque applied at B. Is that true? What law governs this?