The property is described by an equation now known as Hooke's law. Hooke's law says that the restoring force (F) produced by the spring is proportional to the distance by which the spring has been lengthened (x). In equation form, Hooke's law looks like Equation 1: 1  F is the restoring force in newtons (N) 2  k is the spring constant in newtons per meter (N/m) 3  x is the distance the spring has been stretched (or compressed) from its neutral length in meters (m)