Question

Ex 7) Imagine a mile-long bar of metal such as the rail along railroad tracks. Suppose that the rail is anchored on both ends and that, on a hot day, its length expands by 1 foot. If the added length causes the rail to bow upward in a circular arc, about how high would the center of the rail rise above the ground?

Answer 1

9514 1404 393

Answer:

about 44.5 feet

Explanation:

We can write relations for the height of the rail as a function of initial length and expanded length, but the solution cannot be found algebraically. A graphical solution or iterative solution is possible.

Referring to the figure in the second attachment, we can write a relation between the angle value α and the height of the circular arc as ...

h = c·tan(α) . . . . . . where c = half the initial rail length

Then the length of the expanded rail is ...

s = r(2α) = (c/sin(2α)(2α) . . . . . . where s = half the expanded rail length

Rearranging this last equation, we have ...

sin(2α)/(2α) = c/s

It is this equation that must be solved iteratively. We find the solution to be ...

α ≈ 0.0168538794049 radians

So, the height of the circular arc is ...

h = 2640.5·tan(0.0168538794049) ≈ 44.4984550191 . . . feet

The rail will bow upward by about 44.5 feet.

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Additional comments

Note that s and c in the diagram are half the lengths of the arc and the chord, respectively. The ratio of half-lengths is the same as the ratio of full lengths: c/s = 2640/2640.5 = 5280/5281.

We don't know the precise shape the arc will take, but we suspect is is not a circular arc. It seems likely to be a catenary, or something similar.

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We used Newton's method iteration to refine the estimate of the angle from that shown on the graph. The iterator used is x' = x -f(x)/f'(x), where x' is the next guess based on the previous guess of x. Only a few iterations are required obtain an angle value to full calculator precision.