Supporting a Circle

A wire is in the shape of a unit circle centered on the origin, with the portion from θ = 3 0 \theta = 30^\circ to θ = 15 0 \theta = 150^\circ missing (see image). The wire's mass is 1 kg 1 \, \text{kg} , and gravity is 10 m/s 2 10 \, \text{m/s}^2 in the y -y direction.

A force is applied normally to the curve at all points on the curve, such that each unit of curve length experiences an infinitesimal force of the same magnitude:

d F d = α \large\frac{d \, |F|}{d \ell} = \alpha{}

In the above equation, the units on α \alpha are N/m \text{N/m} . If the applied force keeps the wire in stasis, what is the value of α \alpha ?

Note: My intent is to describe a constant linear pressure over the curve length


The answer is 5.7735.

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1 solution

Parth Sankhe
Oct 24, 2018

You take an element of length d l dl and present at angle θ \theta from the negative y axis. Now d l = r d θ dl = rd\theta . (r=1)

The upward force on this element would be d F c o s θ dFcos\theta . Putting d F = α d l dF=\alpha dl and integrating the upward force from 0° to 120°, doubling that to count in the second half, and equalling that to m g mg , would give us the value of α = 10 3 \alpha = \frac {10}{√3} .

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