We can find the effective mass of the spring by finding its kinetic energy. This requires adding all the length elements' kinetic energy, and requires the following integral:

$T=\int_{m} \frac{1}{2} u^{2} d m$

since the spring is uniform, $d m=\left(\frac{d y}{L}\right) m,$ where $L$ is the length of the spring. Hence,

$T =\int_{0}^{L} \frac{1}{2} u^{2}\left(\frac{d y}{L}\right) m$

$=\frac{1}{2} \frac{m}{L} \int_{0}^{L} u^{2} d y$

The velocity of each mass element of the spring is directly proportional to its length, i.e.

$u=\frac{v y}{L},$ from which it follows$:$

${T=\frac{1}{2} \frac{m}{L} \int_{0}^{L}\left(\frac{v y}{L}\right)^{2} d y}$

${=\frac{1}{2} \frac{m}{L^{3}} v^{2} \int_{0}^{L} y^{2} d y}$

${=\frac{1}{2} \frac{m}{L^{3}} v^{2}\left[\frac{y^{3}}{3}\right]_{0}^{L}}$

${=\frac{1}{2} \frac{m}{3} v^{2}}$