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.