I have a structures problem I cannot solve, I have the answer and I have attempted the question. I just need to know where I am going off track. Any assistance would be highly appreciated.
My work is as follows:
At point B:
$T = Fd$, where $d = 300\text{ mm}$ and $F = 100\text{ N}$, therefore $T = 30,000\text{ Nmm}$
From torsion, shear stress $\tau = Tr/J$
$$J = \dfrac{\pi}{2}\cdot(r_1^4 - r_2^4)$$
Therefore $\tau = \dfrac{30,000 \cdot 30 \cdot 2}{\pi(30^4 - 25^4)} = 1.37\text{ MPa}$
This is the first answer listed, which leads me to believe my lecturer listed the answers for (B,A) instead of (A,B)?
However then for point A:
$T = Fd$, I believe $d$ now changes to 330 mm as you have to add the outer radius of the bar so $T = 33,000\text{ Nmm}$
Following the same process used for B:
$$t = \dfrac{33,000 \cdot 30 \cdot 2}{\pi(30^4 - 25^4)} = 1.50\text{ MPa}$$
This is clearly not the answer my lecturer gave. I am a confused as to where I went wrong. I am aware that I did not use the value 250mm but I am unsure where it applies. What makes points A and B different, aside from their distance from the force? I would really appreciate any assistance in the matter, thank you so much for reading.