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## Homework Statement

The question wants me to find the maximum torque.

I do not understand why the solution uses the radius of the smaller part of the shaft rather than the radius of the thicker part of the shaft. Please see the attached image.

## Homework Equations

shear stress = (max torque)(maximum radius)/J where J = polar moment of inertia.

J = π/2c^4 where c = max radius of the rod

## The Attempt at a Solution

I want to use 37.5mm as the radius (radius of larger part of the shaft), but it is not correct. The formula above directly relates torque T and c (radius), which is why I would assume it would be correct to use the largest c value possible. I would appreciate any insight. Thanks.