Here the dependent variable u (x) measures the distance from the equilibrium of the mass situated at x, so that u (x) essentially measures the magnitude of a disturbance (i.e. strain) that is traveling in an elastic material. The resulting force exerted on the mass m at the location x + h is: F Hooke = F x + 2 h − − F x = k [ u ( x + 2 h , t ) − − u ( x + h , t ) ] − − k [ u ( x + h , t ) − − u ( x , t ) ] .