1

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.

closed as off-topic by isherwood, Daniel Griscom, mmathis, The Evil Greebo, Machavity Jun 6 '18 at 20:37

  • This question does not appear to be about home improvement within the scope defined in the help center.
If this question can be reworded to fit the rules in the help center, please edit the question.

  • 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
  • 2
    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
1

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.

Not the answer you're looking for? Browse other questions tagged or ask your own question.