A wire of length $L$ and mass $M$ is fastened around a circular drum and the drum is set into rotation about its centre with constant angular velocity $\omega$. I wanted to find the tension in the string. (Question taken up from Kleppner and Kolenkow Mechanics).

Here is a solutions to this question with a diagram of the $\triangle \theta$ portion of the curve. One thing that baffles me here is why the normal reaction on the wire has not been taken into account.enter image description here Because then the equations would become

$$T \triangle \theta - \triangle N= \triangle m r {\omega}^ 2$$


closed as off-topic by John Rennie, Jon Custer, user36790, Gert, JamalS Oct 22 '16 at 9:31

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  • 2
    $\begingroup$ I agree with you. The problem should have stated that the wire is pre-loaded in tension when it is initially placed on the drum. Then the problem statement would have made sense (at least to me). $\endgroup$ – Chet Miller Oct 21 '16 at 20:49

Check the conditions of the problem. Probably it is assumed that the wire rotates so fast that it expands and leaves contact with the drum. The drum is not really needed. KK only want to find the tension in the hoop which is due to its rotation, not the component due to the normal reaction from the drum.