what means the effective mass of a spring?

When a spring-mass system is set into oscillation, one end of the spring is fixed and the other end is pegged to a mass and oscillates together with the mass.

Thus part of the spring is also oscillating, the mass of the spring that is under oscillation must be taken into account in calculation the period of oscillation.

For a spring-mass system performing Simple Harmonic Motion, it can be shown that about (1/3) of the spring mass can be regarded as in the same oscillation as the mass. This one-third of the spring mass is called the effective mass of the spring.

我相信係當個spring彈出去的時候，個force make 出來的mass.spring每轉一圈的密度越寬，the effective mass of a spring就越大.