- This program represents the string by a discrete mechanical model consisting of N mass points interconnected by N-1 springs. No calculus is needed, and the equations of motion are trivial (but many). Only Hooke’s law relating the force linearly to spring extension is needed, and of course F=ma. A trivial leap-frog numerical scheme is imple-mented, with numerical noise damping. The Fortran code is very simple, but time steps cannot exceed Courant limit.