# Shaft and tension

TSN79
I've got this shaft you can see in my drawing below, and I'm supposed to find the tension at the locations C and D. At point C I use the following equation:

$$\sigma=\frac{M}{W}=\frac{2500\cdot350}{\frac{50^3\cdot\pi}{32}}=71,3 N/mm^2$$

This is apparently the right thing to do. 2500 comes from the reaction forces from the bearings at the ends. What I don't really see is why cant I use the force of 5 kN to calculate this. Putting in 5000*150 instead and exchanging 50 for 60 in the denominator. Could someone explain this to me...?

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If you are referring to the normal stresses due to bending, then it looks like you are using the section modulus in the denominator which is right. The reason you can not do what you are thinking is becasue a force has to be created by a reaction to create the stress. The 5kN load on it's own will do nothing to induce the stress. It is the reactions that create the stress. If you have done beam deflections, you always start at one end where you know the reaction and calculated the moment and shear diagrams moving from end to end. The same thing applys here.