Just click on the green flag and watch the boy launch and fly his kite. The shape of the kite line is called a catenary. A catenary is the curve that a flexible rope, chain or cable, hanging under its own weight, takes on when supported at both ends.
The equation for the catenary is: y = (a/2)(e^(x/a) + e^(-x/a)) The forces acting on the kite and a derivation of the appropriate equations can be found on this NASA web site: https://www.grc.nasa.gov/www/K-12/airplane/kitesag.html
