Finding mass of spring from time period.

[amk_dbz]

As the title suggests, there is a practical in which we have to find mass of the spring (m*) having a block of mass M attached to bottom, from GRAPH of time period (T)^2 against M.
Here's what the practical handbook says:
T^2= [16{(pi)^2}(R^3)N][M+(m*/3)] / [(r^4)n]
there is no reference to what is N,n,R,r
What they do next is to plug in T=0
therefore, M+(m*/3)=0
=> m*/3=-M
Considering magnitude, m*=3(x intercept)

The last part makes some sense but, where does the formula come from?

Thank you! (Sorry for being messy)

 

[andrien]

the formula will come when you will consider kinetic energy of spring(it has mass).see here
http://en.wikipedia.org/wiki/Effective_mass_(spring%E2%80%93mass_system)