Equilibrium  
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Diagram shows a shaft A B with aluminum segment marked 1 of length 1,000 millimeters inside which is a brass shaft marked 2. Both the shafts are centered on the central axis x. A torque T is applied at B. An internal torque T subscript 1 and T subscript 2 is applied on the shafts towards A.

Draw a FBD around end B.

Assume positive internal torques T1 and T2 in shafts (1) and (2).

The equilibrium equation obtained from this FBD is:

∑MX=-T1-T2+T=0      ∴T1+T2=T