Torsion problem with circular shaft

[Precursor]

Homework Statement
[PLAIN]http://img821.imageshack.us/img821/1089/64194000.jpg



The attempt at a solution
How should I get started on this one?

 

[Mapes]

How about drawing a free-body diagram of the shaft, and plotting the torsion as a function of length along the shaft?

 

[pongo38]

use tau/r = T/J = G*theta/L

 

[mugaliens]

How should I get started on this one?

You know the free end (D) is experiencing just 900 N*m of torque, thus, section C-D is only experiencing 900 N*m of torque. Continuing this approach, section B-C is experiencing 900 + 2100 N*m of torque. Continuing this approach, how much torque is section A-B experiencing?

Once you have the torques of all three sections, calculate the sheer stress in each section. Along the way, calculate the angle of twist for each section, and add them up, making sure that for CW torque, it's positive, and for CCW torque, it's negative.