I plan to build a raised planter from 2 courses of concrete blocks. The long dimension is about 9 meters. I staked the corners, and then stretched a string the entire length as taut as I could without pulling up the stakes. The string is horizontal according to the bubble of a line level hanging from any point along the string. How much sag should I assume is in the string compared with e.g. a laser line?

I was hoping to get a practical answer instead of an academic exercise, but assume the modulus of elasticity for cotton is 7.9 GPa, assume the string cross-sectional area is 1E-6 m^2, and assume tension is 80 N.

  • Take measurements with a string bubble at 1/4 and at the center or 1/2 way if you see a difference maybe a cheap laser level would be the way to go, I used string bubble lines 35 years ago but have been using using lasers for 20+ and today they are cheaper than the error in string lines. – Ed Beal May 27 '18 at 5:10
  • How tight is it stretched? – Lee Sam May 27 '18 at 5:55
  • @LeeSam edited. I'd guess 5-10kg of tension. The line level isn't precise enough to measure a difference along the length. – Spencer Joplin May 27 '18 at 15:40
  • As a follow up to Lee Sam's question, what is the spring coefficient of the string? ;-D In other words, it depends entirely on the properties of the string and how tight it is pulled as to how much sag you will get. – statueuphemism May 27 '18 at 16:13
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    I'm voting to close this question as off-topic because it is a physics question, not a home improvement question – mmathis May 29 '18 at 14:57

An eyeball down the string will tell the story, but a proper dryline (not kite string or other weak line) sufficiently taut (as you describe) is not going to have more than 1/4" sag in 30 feet. I'd think that would be suitable for a planter, and you could anticipate the sag when you set your heights and adjust slightly.

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