forces - Conversion between mass units

I am enrolled in a course related to physics, and converting between mass units is so tricky for me. I always treat in my work only in SI units and maybe MKS units, at level of weights, I find it easy.

But what about the mass conversion between these units:

• kg


• kgf.s^2/m

The first I see is that kg comes from N, and kgf.s^2/m comes from kgf, doesn't it? I don't understand, how to convert, for example 50 kg to ()kgf.s^2/m; I couldn't affect it by gravity because they are mass units, not weight-mass conversion anymore.

Please help me.

This is my notes from my class, to get the right units of frequency:

Stiffness   Mass        Frequency
tonf/m      ton.s^2/m   rad/s
kN/m        ton         rad/s
N/m         kg          rad/s
kgf/m       kgf.s^2/m   rad/s

I have always chose the third row to calculate; but maybe in the exam it could come in other units; and I want to understand it.

I don't know if this is so basic that I have to know, but don't get it.