Torque Diagram  
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Free-Body diagram of the shaft A B C which cuts through A B shows the torques 600 Newton meter, 200 Newton meter acting on B and C, and a positive internal torque T subscript 1 acts on A B towards A. The length of A B is 100 millimeters, B C is 300 millimeters.

To find the torque in shaft segment (1), draw a free-body diagram (FBD) that cuts through segment (1) and includes the free end of the shaft.

Sum the torques acting on the FBD:

∑M=-T1+600 N-m -200 N-m=0       ∴ T1=400 N-m