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# Re: Cable Tension Force

• To: "?" <seaint(--nospam--at)seaint.org>
• Subject: Re: Cable Tension Force
• From: Christopher Wright <chrisw(--nospam--at)skypoint.com>
• Date: Mon, 9 Dec 02 09:16:25 -0600

```>I wonder if it is feasible to eliminate sag completely.
No. The statics of the problem are such that it'd take an infinite load. Cables have no bending resistance so any transverse load can only be resisted by a vertical component of cable tension. Accordingly for a vertical component of tension the slope of the cable must be non-zero at the ends, which can only happen if the cable sags, even a little.

Maybe there are still a few old farts on the list who remember clothes lines, particularly the plastic coated wire kind. You'd put them in with turnbuckles so you could take out the sag and keep the sheets from dragging the ground. After a few months they'd be sagging again: Mother complains, I go fix them and repeat the cycle. Mr failed clothesline fixer goes to college and learns to do free body diagrams and it becomes obvious: put a small transverse load on a stretched wire and the wire forms a very shallow V. Balance the transverse load, F, with the vertical component of cable tension, T, and equilibrium requires that the cable tension becomes F/2tanØ where Ø is the small exterior half angle. Doesn't take many cycles of loading to stretch a cable so it sags a bit.

Christopher Wright P.E.    |"They couldn't hit an elephant at
chrisw(--nospam--at)skypoint.com        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/~chrisw

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